asm for memmove on i386 and x86_64

for the sake of simplicity, I've only used rep movsb rather than
breaking up the copy for using rep movsd/q. on all modern cpus, this
seems to be fine, but if there are performance problems, there might
be a need to go back and add support for rep movsd/q.

src/string/x86_64/memmove.s

@@ -0,0 +1,15 @@
+.global memmove
+.type memmove,@function
+memmove:
+	mov %rdi,%rax
+	sub %rsi,%rax
+	cmp %rdx,%rax
+	jae memcpy
+	mov %rdx,%rcx
+	lea -1(%rdi,%rdx),%rdi
+	lea -1(%rsi,%rdx),%rsi
+	std
+	rep movsb
+	cld
+	lea 1(%rdi),%rax
+	ret