x86: do not use i386 code for x86_64 memory functions

arch/x86/lib/string.c contains assembler implementations of memcpy(),
memmove(), and memset() written for i386. Don't use it on x86_64.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>