[gdb/testsuite] Fix gdb.ada/interface.exp with gcc-9

[gdb/testsuite] Fix gdb.ada/interface.exp with gcc-9

When running test-case gdb.ada/interface.exp with gcc-9, we run into:
...
(gdb) info locals^M
s = (x => 1, y => 2, w => 3, h => 4)^M
r = (x => 1, y => 2, w => 3, h => 4)^M
(gdb) FAIL: gdb.ada/interface.exp: info locals
...

The failure is caused by the regexp expecting variable r followed by
variable s.

Fix this by allowing variable s followed by variable r as well.

Tested on x86_64-linux.