modula-2, driver: Handle static-libstd++ for targets without static/dynamic

modula-2, driver: Handle static-libstd++ for targets without static/dynamic

The follows the pattern used in C++ and D drivers to pass -static-libstdc++
onto the target driver to allow spec substitution of static libraries.

NOTE: The handling of Bstatic/dynamic and the possible use of static libgm2
libraries is unimplemented in this driver so far.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/m2/ChangeLog:

	* gm2spec.cc (lang_specific_driver): Pass -static-libstdc++ on to
	the target driver if the linker does not support Bstatic/dynamic.