[gdb/testsuite] Fix gdb.base/break-dbg.exp without xml support

With a gdb build without xml support and test-case gdb.base/break-dbg.exp, I
run into:
...
(gdb) catch syscall^M
warning: Can not parse XML syscalls information; \
  XML support was disabled at compile time.^M
Catchpoint 11 (any syscall)^M
(gdb) FAIL: $exp: catch syscall
...

Fix this by updating the regexp.

Tested on aarch64-linux.

Reviewed-By: Keith Seitz <keiths@redhat.com>