[clang] [Clang] Fix clang to emit llvm-ir for fadd/fsub atomics (PR #162679)

[via cfe-commits]

CarolineConcatto wrote:

@tmatheson-arm  the **_atomicrmw fadd_**  lowers to _**casal**_ without lsfe:
https://godbolt.org/z/e6f1q3YE1

https://github.com/llvm/llvm-project/pull/162679