Conditional compilation of ZGC

[Stefan Karlsson]

Hi all,

This is the first version of the patch to allow the compilation of ZGC 
to be turned on and off. This patch passes tier1, tier2, and hs-tier3 
(except for the test in gcbarriers that need to be updated). This mostly 
shows that we can build ZGC on Linux x86_64 and Solaris Sparc, and that 
the other platforms still build without ZGC. It also shows that we don't 
break the other GCs.

http://cr.openjdk.java.net/~stefank/zgc/zIncludeZGC/webrev.01/

There are a few changes that we want to do for next version:

1) .ad file replace the fatal("ZGC specific code") with 
ShouldNotReachHere(), and maybe assert that we are running with ZGC.

2) Remove the INCLUDE_ZGC from gcCause and gcName, or add INCLUDE_<GC> 
guards for the other GCs.

3) c2_globals.hpp further cleanup. Probably move these flags into a 
gc/z/c2 directory.

Cheers,
StefanK