powerpc: generic nearbyint/nearbyintf

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

	* sysdeps/powerpc/fpu/round_to_integer.h (set_fenv_mode): Add
	NEARBYINT handling.
	* sysdeps/powerpc/fpu/s_nearbyint.c: New file.
	* sysdeps/powerpc/fpu/s_nearbyintf.c: Likewise.
	* sysdeps/powerpc/powerpc32/fpu/s_nearbyint.S: Remove file.
	* sysdeps/powerpc/powerpc32/fpu/s_nearbyintf.S: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/s_nearbyint.S: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/s_nearbyintf.S: Likewise.