This is one more follow-up to PR
https://github.com/llvm/llvm-project/pull/157517 that cleans up the
usage of
libc_errno in math unit-tests (non-smoke). It follows the same rule:

* if you use libc_errno in code literally, include
src/__support/libc_errno.h
* if you only rely on errno constants, include hdr/errno_macros.h

Several tests for exp/log still retain the direct libc_errno usage,
since in some cases they skip doing any validation if the error was
raised. But the direct usage of libc_errno is removed from all other
cases.

https://github.com/llvm/llvm-project/issues/149993

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Fix #98393

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`.

Fixes #86502.

cc @lntue