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`
