RFR: 8213503: Build failure for SerialGC-only JDK

[Kim Barrett]

Please review this small (trivial?) change, fixing some G1GC
conditionalizations so that it is possible to configure and build a
JDK that only provides SerialGC.

CR:
https://bugs.openjdk.java.net/browse/JDK-8213503

Webrev:
http://cr.openjdk.java.net/~kbarrett/8213503/open.00/

Testing:
mach5 tier1 (mostly to verify changes still build normally).
Local build (linux-x64) configured with
  --with-jvm-features=-cmsgc,-epsilongc,-g1gc,-parallelgc,-zgc