PowerPC, fix gdb.base/watchpoint.exp on Power 9

PowerPC, fix gdb.base/watchpoint.exp on Power 9

Test gdb.base/watchpoint.exp generates 4 test errors on Power 9.  The
test uses the test [target_info exists gdb,no_hardware_watchpoints] to
determine if the processor supports hardware watchpoints.  The check
only examines the processor type to determine if it supports hardware
watchpoints.

The PowerPC processors support hardware watchpoints with the
exception of Power 9. The hardware watchpoint support is disabled on
Power 9.  The test skip_hw_watchpoint_tests must be used to correctly
determine if the PowerPC processor supports hardware watchpoints.

This patch replaces the [target_info exists gdb,no_hardware_watchpoints]
with the skip_hw_watchpoint_tests_p check.  With the patch, the test runs
on Power 9 with hardware watchpoint force-disabled.  The test runs on
all other PowerPC processors with and without hardware watchpoints
enabled.

The patch has been tested on Power 9 to verify the test only runs with
hardware breakpoints disabled.  The patch has been tested on X86-64 with
no regression failures.  The test fails on Power 10 due to an internal GDB
error due to resource management.  The resource management issue will be
addressed in another patch.