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

[Roman Kennke]

On Wed, 17 Feb 2021 10:22:22 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
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More pointed fix

Looks good!

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

Marked as reviewed by rkennke (Reviewer).

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