RFR(sh/jdk11): Refactor/isolate critical pinning

[Roman Kennke]

I'm running an effort to bring our upstream exposure vs jdk11u to
(near) zero.

Critical pinning support touches a few places in x86 assembly which are
rather hairy, and should look much better when isolated such that
builds without Shenandoah more obviously generate the same code as
jdk11u upstream currently does.

It moves the new method definitions to Shenandoah-files. It trades
cleaner sharde code at the expense of slightly increased mess in
Shenandoah files (I copied some helper methods there, seems harmless to
me). The new inline code parts are now guarded with #if
INCLUDE_SHENANDOAHGC and if (UseShenandoahGC) so are obviously
guaranteed to not leak out into non-Shenandoah code.

Webrev:
http://cr.openjdk.
java.net/~rkennke/shjdk11-refactor-isolate-critical-pinning/webrev.00/

Testing: hotspot_gc_shenandoah (x86_32, x86_64, builds with and without
Shenandoah)

Ok?

Roman