[flang][runtime] Partial revert of #83383. (#83478)

For `LDBL_MANT_DIG == 113` targets the REAL(16) versions of F18
runtime APIs can stay and should better stay in FortranRuntime.
This way, no additional linking actions are required, because
glibc provides all that is needed.
I thought I would isolate all REAL(16) implementations (both
via `__float128` and `long double`) into Float128Math library,
but that was a bad idea.

This should fix aarch64 buildbots failing gfortran tests.