bug#57109: arm-none-eabi-toolchain atomic support is broken

[Rutherther via Bug reports for GNU Guix <bug-guix@gnu.org>]

> Jean Pierre De Jesus DIAZ via Bug reports for GNU Guix <bug-guix@gnu.org> writes:
> 
> > The following works though. No newlib or anything:
> >
> > guix shell --expression='((@ (gnu packages cross-base) cross-gcc) "arm-none-eabi")' -- \
> >    bash -c "echo '#include <stdatomic.h>' \
> >  | arm-none-eabi-gcc -x c -o /dev/null -c -"
> >
> >
> > —
> > Jean-Pierre De Jesus DIAZ
> >

The reason this works is that the stdatomic.h is used from gcc itself,
instead of the stdatomic.h coming from newlib. It works even with the
arm-none-eabi gcc's coming from `gnu packages embedded`.

The newlib one seems to be broken, though I am unable to find any
discussion on newlib's part. I've tried toolchain coming from
developer.arm.com directly (through nixpkgs that packages the binaries)
and found out that they use stdatomic.h coming from gcc even in full toolchain.

The problem in the Guix ones is that since those use
CROSS_C_INCLUDE_PATH. The paths here get priority to the ones coming from gcc
itself. This then means the newlib stdatomic.h shadows the one coming
from (arm-none-eabi-)gcc:lib.

The toolchain from developer.arm.com does not have this issue, since it
relies on the paths directly from gcc, having the one with newlib as
last. When I try directly including stdatomic.h from newlib, I also
get this error even with that toolchain.

A solution could be to prepend the include path from gcc:lib into the
CROSS_C_INCLUDE_PATH to make it take priority, and prefer the packages
coming from there, or to stop relying on the env vars.

See the toolchain from Nix, the one not using env vars:
```
#include <...> search starts here:
 /nix/store/v9njly0h3i308dgyxzcqb6lqf71q8mkl-gcc-arm-embedded-12.3.rel1/bin/../lib/gcc/arm-none-eabi/12.3.1/include
 /nix/store/v9njly0h3i308dgyxzcqb6lqf71q8mkl-gcc-arm-embedded-12.3.rel1/bin/../lib/gcc/arm-none-eabi/12.3.1/include-fixed
 /nix/store/v9njly0h3i308dgyxzcqb6lqf71q8mkl-gcc-arm-embedded-12.3.rel1/bin/../lib/gcc/arm-none-eabi/12.3.1/../../../../arm-none-eabi/include
```

See Guix one:
```
#include <...> search starts here:
 /gnu/store/h5jxagsf8236ff3lbfcd4yk1qhk2bs4p-profile/arm-none-eabi/include
 /gnu/store/pjc5ysriqs6w99ihkza7xgwm8clfpgp0-gcc-cross-sans-libc-arm-none-eabi-7-2018-q2-update-1.261907-lib/lib/gcc/arm-none-eabi/7.3.1/include
 /gnu/store/pjc5ysriqs6w99ihkza7xgwm8clfpgp0-gcc-cross-sans-libc-arm-none-eabi-7-2018-q2-update-1.261907-lib/lib/gcc/arm-none-eabi/7.3.1/include-fixed
```

Regards,
Rutherther