Parameterize OP_T_THRES from memcopy.h

It moves OP_T_THRES out of memcopy.h to its own header and adjust
each architecture that redefines it.

Checked with a build and check with run-built-tests=no for all major
Linux ABIs.

Co-authored-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

string/memcmp.c

@@ -48,9 +48,6 @@
    and store.  Must be an unsigned type.  */
 # define OPSIZ	(sizeof (op_t))
 
-/* Threshold value for when to enter the unrolled loops.  */
-# define OP_T_THRES	16
-
 /* Type to use for unaligned operations.  */
 typedef unsigned char byte;