[gdb/testsuite] Require hw watchpoint in gdb.ada/task_watch.exp

[gdb/testsuite] Require hw watchpoint in gdb.ada/task_watch.exp

On powerpc64le-linux I run into:
...
(gdb) PASS: gdb.ada/task_watch.exp: info tasks before inserting breakpoint
watch -location value task 3^M
Watchpoint 2: -location value^M
(gdb) PASS: gdb.ada/task_watch.exp: watch -location value task 3
continue^M
Continuing.^M
[Thread 0x7ffff7ccf170 (LWP 65550) exited]^M
[Thread 0x7ffff7abf170 (LWP 65551) exited]^M
FAIL: gdb.ada/task_watch.exp: continue to watchpoint (timeout)
...

On x86_64-linux (where the test-case passes), a hardware watchpoint is used:
...
(gdb) PASS: gdb.ada/task_watch.exp: info tasks before inserting breakpoint
watch -location value task 3^M
Hardware watchpoint 2: -location value^M
...
and after forcing "set can-use-hw-watchpoints 0" we can intermittently
reproduce the same failure.

In the gdb documentation related to watchpoints in multi-threaded programs, we
read:
...
Warning: In multi-threaded programs, software watchpoints have only limited
usefulness.  If GDB creates a software watchpoint, it can only watch the value
of an expression in a single thread.  If you are confident that the expression
can only change due to the current thread’s activity (and if you are also
confident that no other thread can become current), then you can use software
watchpoints as usual.  However, GDB may not notice when a non-current thread’s
activity changes the expression.  (Hardware watchpoints, in contrast, watch an
expression in all threads.)
...

Since the ada task construct is mapped onto threads, it seems that the
same limitation holds for tasks.

Fix this by using skip_hw_watchpoint_tests.

Tested on powerpc64-linux.
Tested-By: Carl Love <cel@us.ibm.com>