gdb Power 9 add test for HW watchpoint support.

The Power 9 processor revision 2.2 has HW watchpoint support disabled due
to a HW bug.  The support is fixed in Power 9 processor revision 2.3.  This
patch add a test to lib/gdb.exp for Power to determine if the processor
supports HW watchpoints or not.  If the Power processor doesn't support HW
watchpoints the proceedure skip_hw_watchpoint_tests will return 1 to
disable the various HW watchpoint tests.

The patch has been tested on Power 9, processor revesions 2.2 and 2.3.  The
patch has also been tested on Power 10.  No regression test failures were
found.