feat(cc): align C compiler vendor detection with the OCaml compiler

[chizy7]

Fixes #14870.

Dune detected the C/C++ compiler vendor with a coarser vocabulary than the OCaml compiler's own OCAML_CC_VENDOR macro (ocaml/aclocal.m4). It recognized only msvc/clang/gcc/other and emitted a "please open an issue" warning for compilers the compiler already names.

What changed

• The detection probe now mirrors OCAML_CC_VENDOR's ordered #if ladder and identifiers: msvc, icc, mingw, clang, gcc, xlc, sunc, unknown.
• cc_vendor now includes: Gcc | Clang | Msvc | Mingw | Intel | Xlc | Sun | Other.

ocamlc -config does not expose c_compiler_vendor, so dune keeps its own preprocessor probe. Only the detection table, parser, and vendor type changed.

Testing

Added a pure-function expect test covering every vendor word. The existing foreign-stubs/cxx-flags.t still passes unchanged.

Two points I'd like a call on

1. mingw-clang link flags now use -lstdc++ instead of -lc++. I believe this is the correct behavior since MinGW ships GNU libstdc++, but wanted to call it out because it changes behavior.
2. xlc/sunc are now recognized but remain flag-neutral. I avoided adding compiler-specific flags that I couldn't test.

[rgrinberg]

Looks good at first glance. @MisterDA or @nojb could you look this over as well?

[nojb]

Does this change have any user-visible effect?

[chizy7]

Mostly no. gcc, clang, msvc, mingw-gcc, and icc all get the same flags as before.

Two exceptions:

1. xlc/sunc used to hit the "please open an issue" warning on every build. They're now recognized, so the warning no longer apears (flags remain unchanged).

2. mingw-clang now links against -lstdc++ instead of -lc++, which is the behavior change I noted above

[nojb]

Not tested, but LGTM