arm: Fix bad memcpy.S str8w macro argument count

Remove the extra (empty) argument passed to str8w, causing the following
error:

   <instantiation>:40:47: error: too many positional arguments
     str8w r0, r3, r4, r5, r6, r7, r8, r9, ip, , abort=19f
                                                 ^
   u-boot/arch/arm/lib/memcpy.S:240:5: note: while in macro instantiation
   17: forward_copy_shift pull=16 push=16
       ^

Note: no functional change intended.

Fixes: d8834a13 ("arm: Use optimized memcpy and memset from linux")
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>