microblaze: Use simple memmove/memcpy implementation from lib/string.c

This is based on previous commit ("microblaze: Use simple memset
implementation from lib/string.c") where generic memset implementation is
used when OPT_LIB_FUNCTION is not defined. The same change can be done for
memset/memcpy implementation where doesn't make sense to have generic
implementation in architecture code.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/1f5cfc026a8a458f3e3134ab80f65bd4ac7e3e8e.1645797329.git.michal.simek@xilinx.com