Implement x86-64 multiarch mempcpy in memcpy

Implement x86-64 multiarch mempcpy in memcpy to share most of code.  It
reduces code size of libc.so.

	[BZ #18858]
	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Remove
	mempcpy-ssse3, mempcpy-ssse3-back, mempcpy-avx-unaligned
	and mempcpy-avx512-no-vzeroupper.
	* sysdeps/x86_64/multiarch/memcpy-avx-unaligned.S (MEMPCPY_CHK):
	New.
	(MEMPCPY): Likewise.
	* sysdeps/x86_64/multiarch/memcpy-avx512-no-vzeroupper.S
	(MEMPCPY_CHK): New.
	(MEMPCPY): Likewise.
	* sysdeps/x86_64/multiarch/memcpy-ssse3-back.S (MEMPCPY_CHK): New.
	(MEMPCPY): Likewise.
	* sysdeps/x86_64/multiarch/memcpy-ssse3.S (MEMPCPY_CHK): New.
	(MEMPCPY): Likewise.
	* sysdeps/x86_64/multiarch/mempcpy-avx-unaligned.S: Removed.
	* sysdeps/x86_64/multiarch/mempcpy-avx512-no-vzeroupper.S:
	Likewise.
	* sysdeps/x86_64/multiarch/mempcpy-ssse3-back.S: Likewise.
	* sysdeps/x86_64/multiarch/mempcpy-ssse3.S: Likewise.

(cherry picked from commit c365e615)