Implement oom handling for MS.

Implement oom handling for MS.

	* sgen-gc.c (collect_nursery): If any object was pinned
	after initial pinning, sort the queue so build_nursery_fragments
	will see the aditional objects.
	If pinning due to OOM happened ask for a major collection.

	* sgen-gc.c (major_do_collection): Adjust the pin queue if
	OOM happened. Here we have to clean discarded entries before
	optimizing the queue or we might end with interior pointers
	in the mix.

	* sgen-gc.c (sgen_collect_major_no_lock): New function so
	major collectors can trigger a collection. They can't
	use public API since the gc lock is not meant to be reentrant.

	* sgen-major-copy-object.h (copy_object_no_checks): Pin
	the object if we can't promote it.

	* sgen-marksweep.c (alloc_obj): Return null if we fail
	to allocate a block for a given freelist size.

	* sgen-marksweep.c (major_alloc_degraded): Ditto.

	* sgen-marksweep.c (major_copy_or_mark_object): Handle OOM.

	* sgen-marksweep.c (major_alloc_small_pinned_obj): If allocation
	fails, try a major collection since this is memory requested by
	the runtime and it's much harder to handle failure.