[libc] Enable hermetic floating point tests

This patch enables us to run the floating point tests as hermetic.
Importantly we now use the internal versions of the `fesetround` and
`fegetround` functions.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D151123

GitOrigin-RevId: 417ea79e792a87d53f5ac4f5388af4b25aa04d7d