[clang] [flang] [llvm] [flang][flang-rt] Add -ffpe-trap= to set the initial FP exception halting mode (PR #208828)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu approved this pull request.

I primarily reviewed the frontend-related code I took a look at the runtime-related bits, and they look reasonable to me, but it would be better if someone else could review those.

https://github.com/llvm/llvm-project/pull/208828