PR modula2/120188: documented example does not work assignvalue m2plugin

PR modula2/120188: documented example does not work assignvalue m2plugin

This patch corrects the gm2 command line used in the documentation
to invoke the m2-plugin.  The patch also includes the documentation
example in dejagnu test code with an expect script to check whether
plugins were enabled.

gcc/ChangeLog:

	PR modula2/120188
	* doc/gm2.texi (Semantic checking): Add -fm2-plugin command line option.

gcc/testsuite/ChangeLog:

	PR modula2/120188
	* lib/gm2-dg.exp (gm2-dg-frontend-configure-check): New function.
	(gm2-dg-runtest): Add -O2 to the option_list.
	* gm2.dg/doc/examples/plugin/fail/assignvalue.mod: New test.
	* gm2.dg/doc/examples/plugin/fail/doc-examples-plugin-fail.exp: New test.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>