[flang] [llvm] [flang] Generate quadmath_wrapper.h for Flang Evaluate. (PR #132817)

[Slava Zakharin via llvm-commits]

vzakhari wrote:

> LGTM for now, but in the mid-term we **must** absolutely avoid having compiler behavior depend on the system configuration where it was compiled. Think of e.g. Debian not having quadmath configured on their build server and because of that no Debian user can compile code with `REAL(16)` in it.

For sure, the compiler needs a library that provides the same functionality as `libquadmath`.  GNU libm provides it, though, not in all versions.  I believe LLVM libm is not complete on this side yet.

https://github.com/llvm/llvm-project/pull/132817