RFR: JDK-8180193: Make marking bitmap code available to other GCs

[Roman Kennke]

This moves marking bitmap code to gc/shared, at least the parts that are
not G1 specific. The only minor functional change is in check_mark():
replace is_in_exact() with is_in_reserved(). If that is not ok, then we
may want to introduce virtual is_in_exact() in CollectedHeap?

Bug:
https://bugs.openjdk.java.net/browse/JDK-8180193
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8180193/webrev.00/

Ok?

Roman