powerpc: fma using builtins

This patch just refactor the assembly implementation to use compiler
builtins instead.

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_fma.S: Remove file.
	* sysdeps/powerpc/fpu/s_fmaf.S: Likewise.
	* sysdeps/powerpc/fpu/s_fma.c: New file.
	* sysdeps/powerpc/fpu/s_fmaf.c: Likewise.