[libc] Small adjustments to fenv tests

Some libcs define non-standard FE_* macros and include them in
FE_ALL_EXCEPT. This change adjusts the fenv tests so that the
non-standard FE_* macros do not interfere when compiled with
fenv.h from another libc.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D95650