[gdb/testsuite] Fix gdb.cp/exceptprint.exp with check-read1

Fix test-case gdb.cp/exceptprint.exp with make target check-read1 by limiting
the output of skip_libstdcxx_probe_tests_prompt by making the used command
more precise: using "info probes stap libstdcxx" instead of "info probes".

Tested on x86_64-linux.