Fix ldbl-128 built-in function use

Fix the following issues with built-in function use in
sysdeps/ieee754/ldbl-128 and sysdeps/ieee754/float128:

* fabsl used __builtin_fabsf128 unconditionally, breaking the build
  with GCC 6 for several architectures; it should use __builtin_fabsl
  with an appropriate redirection in float128_private.h.  (I'm not
  particularly concerned with building glibc with GCC 6; rather, I
  want to be able to run the tgmath.h tests with GCC 6, which is a
  significantly different case for tgmath.h compared to GCC 7 and
  later because of the lack of _FloatN / _FloatNx support in the
  compiler, and at present running the tests with a compiler means
  building glibc with that compiler.)

* Some (conditional) uses of built-in functions had been added to
  ldbl-128 without appropriate float128_private.h remapping (there was
  remapping for the macros controlling whether the built-in functions
  are used, just not for the functions themselves).

* s_llrintl.c called __builtin_round not __builtin_llrintl, which is
  obviously wrong.

Tested with build-many-glibcs.py for aarch64-linux-gnu, GCC 6 (where
it fixes the glibc build) and GCC 12, and with the glibc testsuite for
x86_64.