ldbl-opt: Add err, errx, verr, verrx, warn, warnx, vwarn, and vwarnx (bug 23984)

When support for long double format with 128-bits (-mlong-double-128)
was added for platforms where long double had the same format as double,
such as powerpc, compatibility versions for the functions listed in the
commit title were missed.  Since the older format of long double can
still be used (with -mlong-double-64), using these functions with a
format string that requests the printing of long double variables will
produce wrong outputs.

This patch adds the missing compatibility functions and header magic to
redirect calls to them when -mlong-double-64 is in use.

Tested for powerpc, powerpc64 and powerpc64le.