The string to integer differential fuzzer assumes at least one byte of
meaningful input, but wasn't explicitly checking that. Now it does.

Including src/__suppot/macros/config.h is unnecessary

Made a fuzz test for heap_sort based off of qsort_fuzz implementation

Fixes https://github.com/llvm/llvm-project/issues/100964

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.

- reland #79113
- Fix aarch64 RISC-V build

Reverts llvm/llvm-project#79113
It broke aarch64 build bot machines.

This patch reduces the surface of `FPBits`.