Fix missing sparcv9 --disable-multi-arch fabsl compat symbol (bug 22229).

The --disable-multi-arch case of sparcv9 libm is missing a fabsl
compat symbol for when long double had the same ABI as double.  This
patch adds the missing compat symbol to this implementation.  As my
fix for other instances of this missing compat symbol postdates the
last release, I'm considering this as being part of bug 22229 that was
missing from my previous fix rather than as a separate issue, and so
as not needing a new bug report in Bugzilla.

Tested (compilation only) with build-many-glibcs.py for
sparcv9-linux-gnu --disable-multi-arch.

	[BZ #22229]
	* sysdeps/sparc/sparc32/sparcv9/fpu/s_fabs.S: Include
	<math_ldbl_opt.h>.
	(fabsl): Define as compat symbol at version GLIBC_2_0 for libm.