CGBuiltin: Use frem instruction rather than libcall to implement fmod

AFAICT the semantics of frem match libm's fmod.

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Tom Stellard <tom@stellard.net>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218488 91177308-0d34-0410-b5e6-96231b3b80d8