remove potentially PIC-incompatible relocations from x86_64 and x32 asm

analogous to commit 8ed66ecbcb for i386.

src/string/x86_64/memcpy.s

@@ -1,6 +1,9 @@
 .global memcpy
+.global __memcpy_fwd
+.hidden __memcpy_fwd
 .type memcpy,@function
 memcpy:
+__memcpy_fwd:
 	mov %rdi,%rax
 	cmp $8,%rdx
 	jc 1f

src/string/x86_64/memmove.s

@@ -4,7 +4,8 @@ memmove:
 	mov %rdi,%rax
 	sub %rsi,%rax
 	cmp %rdx,%rax
-	jae memcpy
+.hidden __memcpy_fwd
+	jae __memcpy_fwd
 	mov %rdx,%rcx
 	lea -1(%rdi,%rdx),%rdi
 	lea -1(%rsi,%rdx),%rsi