RFR: JDK-8239081: Shenandoah: Consolidate C1 LRB and native barriers

[Roman Kennke]

This is a fall-out from the recent Lucene debugging session.

Currently, when emitting IN_NATIVE LRB in C1, we generate a simple
runtime call directly in LIR. It'd arguably be more straightforward and
maintainable to simply re-use what we do for regular LRB, with the only
exception to call into a different runtime endpoint from the stub. It
might also be more efficient because it checks heap-stable before
calling into runtime.

If we ever have to backport C1 IN_NATIVE barriers (JDK-8226695) to 11u
(although, we should not strictly need native barriers there right now),
it also means we can skip backporting JDK-8226822, that would no longer
be needed.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8239081
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8239081/webrev.00/

Testing: hotspot_gc_shenandoah (x86_64, x86_32 and aarch64)

Can I please get a review?

Thanks,
Roman