[libc] Add proxy header for fenv.h macro constants.

[lntue]

Context: #87017 #87598

Add a proxy header for fenv.h macro constants so that our implementations and tests will work nicely for both overlay and full build modes.

• Add libc/hdr/fenv_macros.h that will include the system <fenv.h> in overlay mode or include libc/include/llvm-libc-macros/fenv-macros.h in full build mode. (See for instance libc/hdr/math_macros.h ).
• Add its corresponding cmake target in libc/hdr/CMakeLists.txt.
• Replacing as many #include <fenv.h> and cmake target dependency on libc.include.fenv as possible, and document where those inclusion cannot be removed yet (due to usage of types or functions defined in fenv.h)
• Add hdr_fenv_macros target in utils/bazel/llvm-project-overlay/libc/BUILD.bazel.
• Update dependencies to all targets that include hdr/fenv_macros.h.

[llvmbot]

@llvm/issue-subscribers-libc

Author: None (lntue)

Context: https://github.com//pull/87017 https://github.com//pull/87598

Add a proxy header for fenv.h macro constants so that our implementations and tests will work nicely for both overlay and full build modes.

• Add libc/hdr/fenv_macros.h that will include the system <fenv.h> in overlay mode or include libc/include/llvm-libc-macros/fenv-macros.h in full build mode. (See for instance libc/hdr/math_macros.h ).
• Add its corresponding cmake target in libc/hdr/CMakeLists.txt.
• Replacing as many #include <fenv.h> and cmake target dependency on libc.include.fenv as possible, and document where those inclusion cannot be removed yet (due to usage of types or functions defined in fenv.h)
• Add hdr_fenv_macros target in utils/bazel/llvm-project-overlay/libc/BUILD.bazel.
• Update dependencies to all targets that include hdr/fenv_macros.h.

Can you please assign this to me? thanks