powerpc: consolidate rint

This patches consolidates all the powerpc rint{f} implementations on
the generic sysdeps/powerpc/fpu/s_rint{f}.

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/round_to_integer.h (set_fenv_mode,
	round_to_integer_float, round_mode): Add RINT handling.
	(reset_fenv_mode): New symbol.
	* sysdeps/powerpc/fpu/s_rint.c (__rint): Use generic implementation.
	* sysdeps/powerpc/fpu/s_rintf.c (__rintf): Likewise.
	* sysdeps/powerpc/powerpc32/fpu/s_rint.S: Remove file.
	* sysdeps/powerpc/powerpc32/fpu/s_rintf.S: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/s_rint.S: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/s_rintf.S: Likewise.

Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>