ENH: avoid expensive clears in fenv functions

Clearing is 50-100 times more expensive than checking on x86, so check if there
is anything needs to be cleared first. This speeds up scalar operations
by 10%-20%.
Based on Arink Verma code in #3739.

Implement the functions as new C-API functions npy_get_floatstatus and
npy_clear_floatstatus in npy_math.
Merge pull request #3974 from juliantaylor/fenv-improve

ENH: avoid expensive clears in fenv functions