modula-2, plugin: Fix Darwin bootstrap issues.

This corrects some typos in the suffix of the m2rte pluing that
lead to a bootstrap fail on Darwin, where the suffix is not '.so'.

On some versions of Darwin, the linker complains if libSystem is not
linked, so we disable all the default libs, but add libc back.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/m2/ChangeLog:

	* Make-lang.in: Update suffix spellings to use 'soext'.
	Add libc to the plugin link.

(cherry picked from commit 25be11e9)