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

[Roman Kennke]

On Wed, 17 Feb 2021 08:40:17 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> 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

Looks good. I wonder if this should go upstream?

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

Marked as reviewed by rkennke (Reviewer).

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