RFR: Fix builds that do not have Shenandoah or other GCs [v2]

[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

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  More pointed fix

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

Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/16/files
  - new: https://git.openjdk.java.net/shenandoah/pull/16/files/45786b08..22092e91

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=16&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=16&range=00-01

  Stats: 18 lines in 1 file changed: 2 ins; 16 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