[llvm] [FPEnv] Add strictfp attribute to the FP environment manipulation intrinsics (PR #96093)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Maybe we need two attributes, one meaning "this function must be implemented using strictfp operations" and another meaning "The code that calls this function must use strictfp operations"?

I think we need an attribute similar to the memory attribute which covers which aspects of the floating-point environment are read/written. 

https://github.com/llvm/llvm-project/pull/96093