Fix powerpc-nofpu fmaxmagl, fminmagl spurious "invalid" exception (bug 22691).

For soft-float powerpc, fmaxmagl and fminmagl generate spurious
"invalid" exceptions for quiet NaN arguments.  This is another case of
the problems with fabsl inline expansion via comparisons, and so is
fixed by building those functions with -fno-builtin-fabsl.

Tested for powerpc (soft-float).

	[BZ #22691]
	* sysdeps/powerpc/nofpu/Makefile [$(subdir) = math]
	(CFLAGS-s_fmaxmagl.c): New variable.
	[$(subdir) = math] (CFLAGS-s_fminmagl.c: Likewise.