The `FEnvSafeTest.cpp` test fails on AArch64 soft nofp configurations
because LLVM libc does not provide a floating-point environment in these
configurations.

This patch adds another preprocessor guard on `__ARM_FP` to disable the
test on those.

Fix build errors encountered when building math library on WoA.

1. Skip FEnv equality check for MSVC
2. Provide a placeholder type for vector types.

This would ensure that errno value is cleared out before test execution
and tests pass even when LIBC_ERRNO_MODE_SYSTEM_INLINE is specified (and
errno may be clobbered before test execution).

A lot of the tests would fail, however, since errno would end up getting
set to EDOM or ERANGE during test execution and never validated before
the end of the test. This should be fixed - and errno should be
explicitly checked or ignored in all of those cases, but for now add a
TODO to address it later (see open issue #135320) and clear out errno in
test fixture to avoid test failures.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

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.