PR modula2/117411 Request for documentation to include exception example

This patch adds a new section to the gm2 documentation and new
corresponding testcode to the regression testsuite.

gcc/ChangeLog:

	PR modula2/117411
	* doc/gm2.texi (Exception handling): New section.
	(The ISO system module): Add description of COFF_T.
	(Assembler language): Tidy up last sentance.

gcc/testsuite/ChangeLog:

	PR modula2/117411
	* gm2/iso/run/pass/except9.mod: New test.
	* gm2/iso/run/pass/lazyunique.mod: New test.

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