[FPEnv][llvm-reduce] Correct strictfp test.

Correct llvm-reduce strictfp test to follow the rules documented in the
LangRef:
https://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics

This test needed the strictfp attribute added to a function call.

Note that attributes of intrinsics cannot be changed in declarations,
but attributes can be changed in call sites. Thus the changes to the
declarations. And the constrained intrinsics have strictfp attributes
by default.

Test changes verified with D146845.