Disable libunwind for RTEMS OS

[thesummer]

In rust-lang/rust#127021 we added target support for RTEMS OS.

It has a POSIX interface, so we could reuse much of the unix backend, but currently libunwind is not supported.

Add a cfg switch to disable libunwind for RTEMS.

[thesummer]

Seems to depend on #664 merged for the CI errors

@rustbot blocked

[rustbot]

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[workingjubilee]

Your target is panic-abort, why does it even need this?

[workingjubilee]

I'm not opposed to merging, I'm just confused, I wouldn't think this changes anything.

[thesummer]

True, it's weird. Let me check again.

[thesummer]

Ok. So the compiler itself builds fine. However, when I build my simple hello_world application for the target with cargo build -Z build-std=std,panic_abort --target armv7-rtems-eabihf the linking then fails due to unresolved references to libunwind (see below). If I apply the patch it works.

  = note: /tools/lib/gcc/arm-rtems6/13.2.1/../../../../arm-rtems6/bin/ld: /rtems-rust-toolchain/hello_cargo/target/armv7-rtems-eabihf/debug/deps/libstd-409a62e6aece35e6.rlib(std-409a62e6aece35e6.std.7a20a244cb6fd638-cgu.02.rcgu.o): in function `std::backtrace_rs::backtrace::libunwind::Frame::ip':
          /rtems-rust-toolchain/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/src/rust/library/std/src/../../backtrace/src/backtrace/libunwind.rs:44:(.text._ZN3std12backtrace_rs9backtrace9libunwind5Frame2ip17hda06914ae821911cE+0x2c): undefined reference to `_Unwind_GetIP'
          collect2: error: ld returned 1 exit status

[workingjubilee]

Huh. Something feels off here, but for now, merging this is okay!