[clang] [llvm] Enable fexec-charset option (PR #138895)

[Euan Bourke via llvm-commits]

Bourkupine wrote:

Hey Abhina, 
just did some testing on s390x architecture, is it intended that inline assembly is converted to EBCDIC (1047) when the `IBM1047` flag is passed?

I believe gcc skips inline assembly when using `-fexec-charset`, and I'm not sure its a good idea to break the compatibility.

https://github.com/llvm/llvm-project/pull/138895