gdb/testsuite: Add gdb.arch/aarch64-mops-watchpoint.exp

gdb/testsuite: Add gdb.arch/aarch64-mops-watchpoint.exp

Test behaviour of watchpoints triggered by MOPS instructions.  This test
is similar to gdb.base/memops-watchpoint.exp, but specifically for MOPS
instructions rather than whatever instructions are used in the libc's
implementation of memset/memcpy/memmove.

There's a separate watched variable for each set of instructions so that
the testcase can test whether GDB correctly identified the watchpoint
that triggered in each case.

Approved-By: Luis Machado <luis.machado@arm.com>
Tested-By: Luis Machado <luis.machado@arm.com>