[builtins] Avoid enum name conflicts with fenv.h

After a follow-up change (D98332) this header can be included the same time
as fenv.h when running the tests. To avoid enum members conflicting with
the macros/enums defined in the host fenv.h, prefix them with CRT_.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D98333