[compiler-rt][XRay] Make `xray_interface.h` C compliant (#140068)
The XRay interface header uses no C++ specific features aside from using
the `std` namespace and including the C++ variant of C headers. Yet,
these changes prevent using `xray_interface.h` in external tools relying
on C for different reasons. Make this header C compliant by using C
headers, removing the `std` namespace from `std::size_t` and guard
`extern "C"`.
To make sure that further changes to not break the interface
accidentally, port one test from C++ to C. This requires the C23
standard to officially support the attribute syntax used in this test
case.
Note that this only resolves this issue for `xray_interface.h`.
`xray_records.h` is also not C compliant, but requires more work to
port.
Fixes #139902
Signed-off-by: Jan André Reuter <j.reuter@fz-juelich.de>