ldbl-128ibm-compat: Provide ieee128 symbols to narrow functions

Move the narrow math aliasing macros into a new sysdep header file
math-narrow-alias-float128.h.  Then, provide an override header
to supply the necessary changes to supply the *ieee128 aliases of
these symbols.

This adds ieee128 aliases for faddl, fdivl, fmull, fsubl, daddl, ddivl,
dmull, dsubl.