This is a follow-up to #157517, which fixes the remaining math/smoke
tests, and changes them to only include `hdr/errno_macros.h`, as the
tests just need the errno values to pass into `EXPECT_MATH_ERRO` macro.
A couple non-mechanical changes include:

* slightly better test coverage for some exp/log tests -- that actually
validates that errno is set to `EDOM` or `ERANGE` for certain inputs;
* explicit inclusion of `src/__support/libc_errno.h` in
RoundToIntegerTest template, which still needs to be fixed.

With this change there's a single libc_errno usage left:
```
$ grep -lrnH "libc_errno.h" test/src/math/smoke
test/src/math/smoke/RoundToIntegerTest.h
```

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

Add tests for signaling NaNs, and fix function behavior for handling
signaling NaN input.

Fixes https://github.com/llvm/llvm-project/issues/124812

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`.

Reland of #84991

A downstream overlay mode user ran into issues with the isnan macro not
working in our sources with a specific libc configuration. This patch
replaces the last direct includes of math.h with our internal
math_macros.h, along with the necessary build system changes.

Clean up usage of `DECLARE_SPECIAL_CONSTANTS` in global scope.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

Extract non-MPFR math tests into libc-math-smoke-tests.

Reviewed By: sivachandra, jhuber6

Differential Revision: https://reviews.llvm.org/D159477