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.

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

This part of the effort to make all test related pieces into the `test`
directory. This helps is excluding test related pieces in a straight
forward manner if LLVM_INCLUDE_TESTS is OFF. Future patches will also move
the MPFR wrapper and testutils into the 'test' directory.

Apply the formatting rules that were applied to the libc/src directory
to the libc/test directory, as well as the files in libc/utils that are
included by the tests. This does not include automated enforcement.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D116127

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D112673

Some ctype functions are called from other libc functions (e.g. isspace
is used in atoi). By moving ctype_utils.h to __support it becomes easier
to include just the implementations of these functions. For these
reasons the implementation for isspace was moved into
ctype_utils as well.

FPUtils was moved to simplify the build order, and to clarify which
files are a part of the actual libc.

Many files were modified to accomodate these changes, mostly changing
the #include paths.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D107600

Summary:
Having a consistent prefix makes selecting all of the llvm libc tests
easier on any platform that is also using the gtest framework.
This also modifies the TEST and TEST_F macros to enforce this change
moving forward.

Reviewers: sivachandra

Subscribers:

This switches all of the files in src/string, src/math, and
test/src/math from using relative paths (e.g. `#include “include/string.h”`)
to global paths (e.g. `#include <string.h>`) to make bringing up those
functions on other platforms, such as fuchsia, easier.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D91394