[gdb/testsuite] Test skip_libstdcxx_probe_tests in mi-catch-cpp-exceptions.exp

[gdb/testsuite] Test skip_libstdcxx_probe_tests in mi-catch-cpp-exceptions.exp

On a system without SDT probes in libstdc++, we run into:
...
FAIL: gdb.mi/mi-catch-cpp-exceptions.exp: all with invalid regexp: run until \
  breakpoint in main (unknown output after running)
...

The test-case uses a regexp argument for the catch throw/rethrow/catch
command, which is only supported on systems with SDT probes in libstdc++.

Fix this by marking the portions of the test-case that use a regexp argument
as unsupported on a system without SDT probes.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-07-25  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24830
	* gdb.mi/mi-catch-cpp-exceptions.exp: Call
	mi_skip_libstdcxx_probe_tests, and skip unsupported tests.
	* lib/gdb.exp (skip_libstdcxx_probe_tests_prompt): Factor out of ...
	(skip_libstdcxx_probe_tests): ... here.
	* lib/mi-support.exp (mi_skip_libstdcxx_probe_tests): New proc.