[FPEnv][TableGen] Add strictfp attribute to constrained intrinsics by default.

In D146869 @arsenm pointed out that the constrained intrinsics aren't
getting the strictfp attribute by default. They should be since they are
required to have it anyway.

TableGen did not know about this attribute until now. This patch adds
strictfp to TableGen, and it uses it on all of the constrained intrinsics.

Differential Revision: https://reviews.llvm.org/D154991