fixup! Export optimised __memcpy

Sinc [1] uaccess_with_memcpy.c uses __memcpy instead of memcpy. Make
the optimised memcpy available as __memcpy as well to avoid linkage
failures.

[1] ceac10c8 ("ARM: 9290/1: uaccess: Fix KASAN false-positives")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>