[libc] Enable hermetic floating point tests again.

[libc] Enable hermetic floating point tests again.

Fixing an issue with LLVM libc's fenv.h defined rounding mode macros
differently from system libc, making get_round() return different values from
fegetround().  Also letting math tests to skip rounding modes that cannot be
set.  This should allow math tests to be run on platforms in which fenv.h is not
implemented yet.

This allows us to re-enable hermatic floating point tests in
https://reviews.llvm.org/D151123 and reverting https://reviews.llvm.org/D152742.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D152873