libquadmath: Restore linking against -lm on most targets [PR112963]

The r14-4825 change added AC_CHECK_LIBM to libquadmath configure.ac and
replaced unconditional linking with -lm with linking with $(LIBM)
determined by that.
Unfortunately that broke bare metal targets because AC_CHECK_LIBM attempts
to link against -lm and this was after (unconditional) GCC_NO_EXECUTABLES.
Then r14-4863 partially reverted that change (no longer AC_CHECK_LIBM),
but didn't revert the Makefile.am change of -lm to $(LIBM), which had
the effect that libquadmath is not linked against -lm on any arch.
That is a serious problem though e.g. on Linux, because libquadmath calls
a few libm entrypoints and e.g. on powerpc64le the underlinking can cause
crashes in IFUNC resolvers of libm.
Instead of adding further reversion of the r14-4825 commit and use -lm
unconditionally again, this patch adds an AC_CHECK_LIBM like substitutions
with the *-ncr-sysv4.3* target handling removed (I think we don't support
such targets, especially not in libquadmath) and with the default case
replaced by simple using -lm.  That is something in between using -lm
unconditionally and what AC_CHECK_LIBM does if it would work on bare metal
- we know from GCC 13 and earlier that we can link -lm on all targets
libquadmath is built for, and just white list a couple of targets which
we know don't have separate -lm and don't want to link against that
(like Darwin, Cygwin, ...).

2023-12-12  Jakub Jelinek  <jakub@redhat.com>

	PR libquadmath/112963
	* configure.ac (LIBM): Readd AC_CHECK_LIBM-like check without doing
	AC_CHECK_LIB in it.
	* configure: Regenerated.
	* Makefile.in: Regenerated.