RFR: Properly update roots in code cache

[Roman Kennke]

With concurrent code cache evacuation, we now have to make sure we
properly update refs in the code cache at the start of full-gc. The
SCM::update_roots() only asserts that code cache roots point to
to-space, which is not correct under conc codecache evac. This patch
changes the code so that it properly updates code cache roots in full GC:

http://cr.openjdk.java.net/~rkennke/fixfullupdatecodecache/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/fixfullupdatecodecache/webrev.00/>

Fixes the remaining issues that I found with WeakReferenceTest in jcstress.

Roman