watchpoint-reuse-slot.exp: Correctly skip unsupported commands.

watchpoint-reuse-slot.exp: Correctly skip unsupported commands.

The test case "watchpoint-reuse-slot.exp" yields a lot of failures on
s390/s390x: all instances of awatch, rwatch, and hbreak are performed
even though they aren't supported on these targets.  This is because
the test case ignores non-support error messages when probing for
support of these commands, like:

    (gdb) rwatch buf.byte[0]
    Target does not support this type of hardware watchpoint.

The patch adds handling for this case in the appropriate
gdb_test_multiple invocations.

gdb/testsuite/
	* gdb.base/watchpoint-reuse-slot.exp: Handle the case that the
	target lacks support for awatch, rwatch, or hbreak.