powerpc: Use generic fabs{f} implementations

Since be2e25bb the generic ieee754 implementation uses
compiler builtin which generates fabs{f} for all supported targets.

Checked on powerpc-linux-gnu (built without --with-cpu, with
--with-cpu=power4 and with --with-cpu=power5+ and --disable-multi-arch),
powerpc64-linux-gnu (built without --with-cp and with --with-cpu=power5+
and --disable-multi-arch).

	* sysdeps/powerpc/fpu/s_fabs.S: Remove file.
	* sysdeps/powerpc/fpu/s_fabsf.S: Likewise.