[CodeGen] fix const-ness of cbrt and fma

cbrt() is always constant because it can't overflow or underflow. Therefore, it can't set errno.

fma() is not always constant because it can overflow or underflow. Therefore, it can set errno.
But we know that it never sets errno on GNU / MSVC, so make it constant in those environments.

Differential Revision: https://reviews.llvm.org/D39641

llvm-svn: 318093