[libcxx] [llvm] [libc++] Add support for picolibc and newlib in RUNTIMES_USE_LIBC (PR #147956)

[Volodymyr Turanskyy via llvm-commits]

voltur01 wrote:

Hi, thank you for highlighting these! Do you see any failures or other issues related to the checks?

I believe that both checks should work as expected, since they include a header file and if the header file comes from picolibc, indeed, the check will show.

I am not sure why the second check results in setting `LIBCXX-PICOLIBC-FIXME` - some spot-checks show that this is used to mark tests as unsupported when picolibc does not provide a feature, so I am not sure how to _fix_ these, other than extend picolibc.

https://github.com/llvm/llvm-project/pull/147956