[gdb/testsuite] Fix ERROR in gdb.base/watchpoint-unaligned.exp

[gdb/testsuite] Fix ERROR in gdb.base/watchpoint-unaligned.exp

In PR23888 an error is reported:
...
ERROR: tcl error sourcing watchpoint-unaligned.exp.
ERROR: expected boolean value but got ""
    while executing
"if {$wpnum} {
...

This presumably happens when:
- skip_hw_watchpoint_tests returns 0 meaning hw watchpoints are supported
- gdb fails to set a hw watchpoint and instead sets a sw watchpoint

That particular situation is handled for arm:
...
    -re "Watchpoint (\[0-9\]+): .*\r\n$gdb_prompt $" {
        if {[istarget "arm*-*-*"]} {
            untested $test
            set wpnum 0
        }
    }
...
but not for any other targets so wpnum remains "", triggering the ERROR.

Possibly this has been fixed for powerpc by commit 8d4e4d13afb ("gdb Power 9
add test for HW watchpoint support."), but it's still possible for other
targets.

Fix this by:
- initializing wpnum to 0 instead of ""
- signalling the failure to set a hw watchpoint by a fail

Tested on x86_64-linux, also by adding:
...
gdb_test_no_output "set can-use-hw-watchpoints 0"
...
and verifying that it triggers the fail.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23888