[gdb/testsuite] Fix gdb.ada/catch_ex_std.exp with remote-gdbserver-on-localhost

[gdb/testsuite] Fix gdb.ada/catch_ex_std.exp with remote-gdbserver-on-localhost

When running test-case gdb.ada/catch_ex_std.exp on target board
remote-gdbserver-on-localhost, I run into:
...
(gdb) continue^M
Continuing.^M
[Inferior 1 (process 15656) exited with code 0177]^M
(gdb) FAIL: gdb.ada/catch_ex_std.exp: runto: run to main
Remote debugging from host ::1, port 49780^M
/home/vries/foo: error while loading shared libraries: libsome_package.so: \
  cannot open shared object file: No such file or directory^M
...

Fix this by adding the usual shared-library + remote-target helper
"gdb_load_shlib $sofile".

Tested on x86_64-linux with native and target board
remote-gdbserver-on-localhost.