Add __builtin_set_flt_rounds

This builtin will be converted to llvm.set.rounding intrinsic
in IR level and should be work with "#pragma STDC FENV_ACCESS ON"
since it changes default FP environment. Users can change rounding
mode via this builtin without introducing libc dependency.

Reviewed by: andrew.w.kaylor, rjmccall, sepavloff, aaron.ballman
Differential Revision: https://reviews.llvm.org/D145765


Signed-off-by: jinge90 <ge.jin@intel.com>