RFR: Fix builds that do not have Shenandoah or other GCs

[Aleksey Shipilev]

Lots of builds are currently failing with:



     if (strcmp(ShenandoahGCMode, "generational") == 0) {
                ^~~~~~~~~~~~~~~~

The new code in `gcConfiguration.cpp` should actually sense which configurations are available.

Additional testing:
 - [x] Linux PPC64 build (as one failing example), now passing
 - [x] Linux x86_64 `gc/shenandoah/generational` tests

-------------

Commit messages:
 - Fix builds that do not have Shenandoah or other GCs

Changes: https://git.openjdk.java.net/shenandoah/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=16&range=00
  Stats: 17 lines in 1 file changed: 17 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/16/head:pull/16

PR: https://git.openjdk.java.net/shenandoah/pull/16