tools/nolibc: x86_64: use local label in memcpy/memmove

Compiling arch-x86_64.h with clang and binutils LD yields
duplicate label errors:

.../gcc-13.2.0-nolibc/x86_64-linux/bin/x86_64-linux-ld: error: LLVM gold plugin: <inline asm>:44:1: symbol '.Lbackward_copy' is already defined
.Lbackward_copy:leaq -1(%rdi, %rcx, 1), %rdi

Instead of a local symbol use a local label which can be defined
multiple times and therefore avoids the error.

Reviewed-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240812-nolibc-lto-v2-3-736af7bbefa8@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>