gdb/testsuite: Add gdb.base/memops-watchpoint.exp

Test behaviour of watchpoints triggered by libc's memset/memcpy/memmove.
These functions are frequently optimized with specialized instructions
that favor larger memory access operations, so make sure GDB behaves
correctly in their presence.

There's a separate watched variable for each function so that the testcase
can test whether GDB correctly identified the watchpoint that triggered.

Also, the watchpoint is 28 bytes away from the beginning of the buffer
being modified, so that large memory accesses (if present) are exercised.

PR testsuite/31484
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31484



Approved-by: Kevin Buettner <kevinb@redhat.com>