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
```

This PR adds fmodbf16 basic math function for BFloat16 type along with
the tests.

---------

Signed-off-by: Krishna Pandey <kpandey81930@gmail.com>

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

Make sure to clear out all FE_ALL_EXCEPT bits both before and after
invoking function under test -- otherwise the very first check for the
exception bits in the unit test may fail due to bits set prior to test
invocation.

Fixes build and tests with compiler-rt on x86.

Fixes #98709.

Part of #93566.

This adds a new test fixture class FEnvSafeTest (usable as a base
class for other fixtures) that ensures each test doesn't perturb
the `fenv_t` state that the next test will start with.  It also
provides types and methods tests can use to explicitly wrap code
under test either to check that it doesn't perturb the state or
to save and restore the state around particular test code.

All the fenv and math tests are updated to use this so that none
can affect another.  Expectations that code under test and/or
tests themselves don't perturb state can be added later.

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.