[clang] [CIR][X86] Implement generic fallback for x86 builtins (PR #177639)

[Priyanshu Kumar via cfe-commits]

Priyanshu3820 wrote:

> It's not clear to me what value this is adding. It seems like a lot of indirection to accomplish something that is already being done more simply.

earlier, emitx86BuiltinExpr returned NYI for `rdpmc`, which is also the only x86 builtin not handled in `CiRGenBuiltinX86.cpp`. We also don't have specific handlers for many architectures like arm, so a similar approach for arm would get many of the builtins working in those architectures which currently crashes because currently `TargetCXXABI::GenericARM` is marked NYI()(Which I don't understand why since it can just use itanium abi just like classic codegen)

https://github.com/llvm/llvm-project/pull/177639