[libc] Add proxy header math_macros.h. (#87598) Context: https://github.com/llvm/llvm-project/pull/87017 - Add proxy header libc/hdr/math_macros.h that will: - include <math.h> in overlay mode, - include "include/llvm-libc-macros/math-macros.h" in full build mode. - Its corresponding CMake target libc.hdr.math_macros will only depend on libc.include.math and libc.include.llvm-libc-macros.math_macros in full build mode. - Replace all #include "include/llvm-libc-macros/math-macros.h" with #include "hdr/math_macros.h". - Add dependency to libc.hdr.math_macros CMake target when using add_fp_unittest. - Update the remaining dependency. - Update bazel overlay: add libc:hdr_math_macros target, and replacing all dependency on libc:llvm_libc_macros_math_macros with libc:hdr_math_macros.