[clang] [flang] [RFC][flang][runtime] Add FortranFloat128Math wrapper library. (PR #81971)

[Slava Zakharin via cfe-commits]

vzakhari wrote:

I have been doing some renaming and restructuring before uploading this for review, and I did not test the final version with `libquadmath`.  The latest commit makes the `-DFLANG_RUNTIME_F128_MATH_LIB=libquadmath` compiler fully functional (the end-to-end test is working).

Thank you all for the reviews! I am going to merge this as soon as CI completes.

I decided to keep the `compiler is built without support for` message as is, because emitting NYI might be confusing to the compiler developers.  For users it will look like a compiler error, so I guess it does not matter for them if it has the NYI prefix or not.

https://github.com/llvm/llvm-project/pull/81971