Add VZEROUPPER to memset-vec-unaligned-erms.S [BZ #21081]

Since memset-vec-unaligned-erms.S has VDUP_TO_VEC0_AND_SET_RETURN at
function entry, memset optimized for AVX2 and AVX512 will always use
ymm/zmm register. VZEROUPPER should be placed before ret in

L(stosb):
        movq    %rdx, %rcx
        movzbl  %sil, %eax
        movq    %rdi, %rdx
        rep stosb
        movq    %rdx, %rax
        ret

since it can be reached from

L(stosb_more_2x_vec):
        cmpq    $REP_STOSB_THRESHOLD, %rdx
        ja      L(stosb)

	[BZ #21081]
	* sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
	(L(stosb)): Add VZEROUPPER before ret.

(cherry picked from commit 02b78ff7)