[libc] Add platform independent floating point rounding mode checks.

[libc] Add platform independent floating point rounding mode checks.

Many math functions need to check for floating point rounding modes to
return correct values.  Currently most of them use the internal implementation
of `fegetround`, which is platform-dependent and blocking math functions to be
enabled on platforms with unimplemented `fegetround`.  In this change, we add
platform independent rounding mode checks and switching math functions to use
them instead. https://github.com/llvm/llvm-project/issues/63016

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D152280