RFR (M): 8037915: PPC64/AIX: Several smaller fixes

[Lindenmaier, Goetz]

Hi,

Please review and test the following change.  I also need a sponsor, please.
http://cr.openjdk.java.net/~goetz/webrevs/8037915/webrev.00/

The change contains a row of fixes that only affects ppc64 or aix subdirs.
- Improve checks for supported instructions.
- Enable TrapBasedRangeChecks, was disabled by accident.
- Remove some useless NULL checks.
- Adapt includes for 8003935: Simplify the needed includes for using Thread::current()
- Add missing cast in load_const_optimized(). Asserts on aix.
- Port 8014555: G1: Memory ordering problem with Conc refinement and card marking to ppc

The change also needs to go to jdk8u or ppc-aix-port/stage.

Thanks and best regards,
  Goetz.