Fix hardware watchpoint check in test gdb.base/watchpoint-reuse-slot.exp

Fix hardware watchpoint check in test gdb.base/watchpoint-reuse-slot.exp

This test generates 48 failures on Power 9 when testing with HW watchpoints
enabled.  Note HW watchpoint support is disabled on Power 9 due to a HW bug.
The skip_hw_watchpoint_tests proc must be used to correctly determine
if the processor supports HW watchpoints.

This patch replaces the [target_info exists gdb,no_hardware_watchpoints]
with the skip_hw_watchpoint_tests check.

This patch was tested on Power 9, Power 10 and X86-64 with no regressions.