fix: provide newlib syscall stubs for GCC 15+ toolchains

[Fabian Gottstein (fabiangottstein)]

Newlib-nano does not implement the POSIX OS-interface subroutines (_close, _fstat, _getpid, _isatty, _kill, _lseek, _read, _write); they must be supplied by the application, a BSP, or via --specs=nosys.specs. The newlib manual lists these explicitly as stubs that a bare-metal target has to provide to allow libc to link [1][2].

libstdc++ enables precondition assertions via _GLIBCXX_ASSERTIONS, which is defined by default in unoptimized (Debug) builds [3]. With the bumped toolchain (arm-none-eabi-gcc 15) a failed assertion is reported through std::__glibcxx_assert_fail, whose implementation calls fprintf. This newly drags newlib's stdio backend into the image, which references the reentrant covers _read_r, _write_r, _close_r, etc.; those in turn reference the underlying _close, _fstat, _isatty, _lseek, _read and _write. Because the image did not pull in stdio before, the missing stubs went unnoticed; now linking fails with "undefined reference to _close/_lseek/_read/_write/_fstat/_isatty" and the accompanying "dangerous relocation: unsupported relocation" errors (caused by the undefined symbols resolving to absolute 0).

Add SysCallStubs.cpp providing weak, ENOSYS-returning implementations of these syscalls so the image links regardless of what drags in stdio. _getpid and _kill are included defensively (e.g. for the abort/raise path) though not currently referenced. The stubs are weak so applications can still override them with real implementations, and are only compiled for GCC 15 and newer.

[1] Newlib manual, "System Calls":
https://sourceware.org/newlib/libc.html#Syscalls
[2] Newlib manual, per-function "Supporting OS subroutines required"
(e.g. assert, printf/fprintf): https://sourceware.org/newlib/libc.html
[3] libstdc++ manual, "Macros" (_GLIBCXX_ASSERTIONS):
https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_macros.html

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	7		0	0	0.27s
✅ ACTION	zizmor	7	0	0	0	2.55s
✅ CPP	clang-format	15	2	0	0	0.36s
✅ CPP	cppcheck	15		0	0	0.72s
✅ DOCKERFILE	hadolint	1		0	0	0.02s
✅ JSON	jsonlint	7		0	0	0.39s
✅ JSON	prettier	7	2	0	0	0.57s
⚠️ MARKDOWN	markdownlint	6	1	10	0	0.99s
✅ MARKDOWN	markdown-table-formatter	6	1	0	0	0.39s
✅ REPOSITORY	checkov	yes		no	no	47.02s
✅ REPOSITORY	git_diff	yes		no	no	0.2s
✅ REPOSITORY	grype	yes		no	no	75.89s
✅ REPOSITORY	ls-lint	yes		no	no	0.01s
✅ REPOSITORY	osv-scanner	yes		no	no	0.93s
✅ REPOSITORY	secretlint	yes		no	no	85.5s
✅ REPOSITORY	syft	yes		no	no	1.89s
✅ REPOSITORY	trivy	yes		no	no	21.82s
✅ REPOSITORY	trivy-sbom	yes		no	no	1.97s
✅ REPOSITORY	trufflehog	yes		no	no	18.14s
⚠️ SPELL	lychee	34		2	0	18.81s
✅ YAML	prettier	10	1	0	0	0.6s
✅ YAML	v8r	10		0	0	7.52s
✅ YAML	yamllint	10		0	0	0.54s

Detailed Issues

⚠️ SPELL / lychee - 2 errors

📝 Summary
---------------------
🔍 Total..........249
🔗 Unique.........237
✅ Successful.....245
⏳ Timeouts.........0
🔀 Redirected.....114
👻 Excluded.........0
❓ Unknown..........0
🚫 Errors...........2
⛔ Unsupported......2

Errors in .github/ISSUE_TEMPLATE/add-or-update-hal-driver.md
[ERROR] http://mcd.rou.st.com/modules.php?name=mcu (at 28:60) | Connection failed. Check network connectivity and firewall settings

Errors in README.md
[ERROR] https://st.com/ (at 5:97) | HTTP/2 protocol error. Server may not support HTTP/2 properly | Followed 1 redirect. Redirects: https://st.com/ --[301]--> https://www.st.com/

Hint: Followed 114 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Use verbose mode (`-v`/`-vv`) to see redirection details.

⚠️ MARKDOWN / markdownlint - 10 errors

.github/ISSUE_TEMPLATE/add-or-update-hal-driver.md:10 error MD025/single-title/single-h1 Multiple top-level headings in the same document [Context: "How to import a HAL driver"]
CHANGELOG.md:83 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "⚠ BREAKING CHANGES"]
CHANGELOG.md:87 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Features"]
CHANGELOG.md:94 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Bug Fixes"]
CHANGELOG.md:103 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Features"]
CHANGELOG.md:122 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Bug Fixes"]
CHANGELOG.md:140 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "⚠ BREAKING CHANGES"]
CHANGELOG.md:144 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Features"]
CHANGELOG.md:151 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Features"]
CHANGELOG.md:162 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Bug Fixes"]

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[Fabian Gottstein (fabiangottstein)]

@philips-software/awesome-embedded-projects
An alternative fix is to avoid pulling fprintf into the image by disabling the assertions for embedded targets in Debug configuration (define _GLIBCXX_NO_ASSERTIONS, which prevents the implicit definition of _GLIBCXX_ASSERTIONS in unoptimized builds).