This PR first adds osutils for Windows, and changes some libc code to
make libc and its tests build on the Windows target. It then temporarily
disables some libc tests that are currently problematic on Windows.

Specifically, the changes besides the addition of osutils include:

- Macro `LIBC_TYPES_HAS_FLOAT16` is disabled on Windows. `clang-cl`
generates calls to functions in `compiler-rt` to handle float16
arithmetic and these functions are currently not linked in on Windows.
- Macro `LIBC_TYPES_HAS_INT128` is disabled on Windows.
- The invocation to `::aligned_malloc` is changed to an invocation to
`::_aligned_malloc`.
- The following unit tests are temporarily disabled because they
currently fail on Windows:
  - `test.src.__support.big_int_test`
  - `test.src.__support.arg_list_test`
  - `test.src.fenv.getenv_and_setenv_test`
- Tests involving `__m128i`, `__m256i`, and `__m512i` in
`test.src.string.memory_utils.op_tests.cpp`
- `test_range_errors` in `libc/test/src/math/smoke/AddTest.h` and
`libc/test/src/math/smoke/SubTest.h`

This manifests as `AddressSanitizer: stack-use-after-return` w/o this
change. The `~CheckFEnv()` method of checking fenv seems to only work
for test fixtures.

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.

Fixes #88187

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.

This patch primarily fixes the fenv implementation on Windows, since
Windows uses the MXCSR in place of the x87 status registers for storing
information about the floating point environment. This allows FEnv to
work correctly on Windows, and successfully build.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D121839

Remove the redundant header FPUtil/FEnvUtils.h, use FPUtil/FEnvImpl.h header instead.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D120965

This patch applies the lint rules described in the previous patch. There
was also a significant amount of effort put into manually fixing things,
since all of the templated functions, or structs defined in /spec, were
not updated and had to be handled manually.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D114302

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D110611