[gdb/testsuite] Fix gdb.rust/watch.exp on ppc64le

[gdb/testsuite] Fix gdb.rust/watch.exp on ppc64le

On x86_64-linux, I have:
...
(gdb) watch -location y^M
Hardware watchpoint 2: -location y^M
(gdb) PASS: gdb.rust/watch.exp: watch -location y
...
but on powerpc64le-linux, I run into:
...
(gdb) watch -location y^M
Watchpoint 2: -location y^M
(gdb) FAIL: gdb.rust/watch.exp: watch -location y
...
due to the regexp matching "Hardware watchpoint" but not "Watchpoint":
...
gdb_test "watch -location y" ".*watchpoint .* -location .*"
...

Fix this by making the regexp less restrictive.

Tested on x86_64-linux and powerpc64le-linux.