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

Implemented few entry points for REAL(16) math in FortranF128Math
static library. It is a thin wrapper around GNU libquadmath.
Flang driver can always link it, and the dependencies will
be brought in as needed.
The final Fortran program/library that uses any of the entry points
will depend on the underlying third-party library - this dependency
has to be resolved somehow. I added FLANG_RUNTIME_F128_MATH_LIB
CMake control so that the compiler driver and the runtime library
can be built using the same third-party library: this way the linker
knows which dependency to link in (under --as-needed).
The compiler distribution should specify which third-party library
is required for linking/running the apps that use REAL(16).
The compiler package may provide a version of the third-party library
or at least a stub library that can be used for linking, but
the final program execution will still require the actual library.