x86_64 defines either memcpy or __memcpy depending on the gcc version, and
it looks like UML needs to follow that in its exporting.

Cc: Gabriel C <nix.or.die@googlemail.com>
Signed-off-by: NJeff Dike <jdike@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>