[tests] Test for GC descriptors - first draft.

This runs for a long time and potentially uses lots of memory.  It's not integrated
in the test suite and must be run manually.
[tests] Optional big-switch version of GC descriptor test.

This takes ages to compile with mcs.  See bug #10781.
[tests] Test valuetype GC descriptors, too.
Merge branch 'feature-descriptor-tests'
[sgen] Disable parallel M&S in combination with split nursery.

It crashes at least sgen-weakref-test.exe, and there's more important stuff to
attend to.
Dist the Makefile.am.in files. Fixes #10874.
Runtime assembly mapping for PCL.
Merge branch 'master' into pcl-integration2