PowerPC, fix test gdb.base/watchpoint-stops-at-right-insn.exp

PowerPC, fix test gdb.base/watchpoint-stops-at-right-insn.exp

Test gdb.base/watchpoint-stops-at-right-insn.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.  Note, the test works fine on Power 10.

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 is
disabled on Power 9 but runs on all other PowerPC processors.

The patch has been tested on Power 9, Power 10 and X86-64 with no
regression failures.