PR modula2/108119 disable m2rte plugin by default

This patch disables the m2rte plugin by default.  The driver
will only append the -fplugin=m2rte command line option for cc1gm2
if -fm2-plugin is present.  It only enabled providing ENABLE_PLUGIN
is defined.  gcc/m2/Make-file.in will only build and install m2rte
if enable_plugin is yes.

gcc/m2/ChangeLog:

	PR modula2/108119
	* Make-lang.in (M2RTE_PLUGIN_SO): Assigned to
	plugin/m2rte$(exeext).so if enable_plugin is yes.
	(m2.all.cross): Replace plugin/m2rte$(soext) with
	$(M2RTE_PLUGIN_SO).
	(m2.all.encap): Replace plugin/m2rte$(soext) with
	$(M2RTE_PLUGIN_SO).
	(m2.install-plugin): Add dummy rule when enable_plugin
	is not yes.
	(plugin/m2rte$(exeext).so): Add dummy rule when enable_plugin
	is not yes.
	(m2/stage2/cc1gm2$(exeext)): Replace plugin/m2rte$(soext) with
	$(M2RTE_PLUGIN_SO).
	(m2/stage1/cc1gm2$(exeext)): Replace plugin/m2rte$(soext) with
	$(M2RTE_PLUGIN_SO).
	* gm2spec.cc (lang_specific_driver): Set need_plugin to false
	by default.

gcc/testsuite/ChangeLog:

	PR modula2/108119
	* gm2/iso/check/fail/iso-check-fail.exp (gm2_init_iso): Add -fm2-plugin.
	* gm2/switches/auto-init/fail/switches-auto-init-fail.exp
	(gm2_init_iso): Add -fm2-plugin.
	* gm2/switches/check-all/pim2/fail/switches-check-all-pim2-fail.exp
	(gm2_init_pim2): Add -fm2-plugin.
	* gm2/switches/check-all/plugin/iso/fail/switches-check-all-plugin-iso-fail.exp
	(gm2_init_iso): Add -fm2-plugin.
	* gm2/switches/check-all/plugin/pim2/fail/switches-check-all-plugin-pim2-fail.exp
	(gm2_init_pim2): Add -fm2-plugin.

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