Optimize mempcpy on sparc.

Optimize mempcpy on sparc.

	* sysdeps/sparc/sparc32/memcpy.S: Implement mempcpy using a stub
	that branches into memcpy.
	* sysdeps/sparc/sparc64/memcpy.S: Likewise.
	* sysdeps/sparc/sparc64/multiarch/memcpy-niagara1.S: Likewise.
	* sysdeps/sparc/sparc64/multiarch/memcpy-niagara2.S: Likewise.
	* sysdeps/sparc/sparc64/multiarch/memcpy-ultra3.S: Likewise.
	* sysdeps/sparc/sparc64/multiarch/memcpy.S: Add mempcpy multiarch
	bits.
	* sysdeps/sparc/sparc64/rtld-memcpy.c: Include generic mempcpy
	implementation too.
	* sysdeps/sparc/mempcpy.S: New file.