Properly message out that shenandoah is disabled

[judovana]

Unluckily, ShenandoahGC in jdk11 i still disabled by default. In addition, its output is broken:

checking for dlopen in -ldl... yes
checking if shenandoah can be built... checking if zgc can be built... no, platform not supported
checking if jvmci module jdk.internal.vm.ci should be built... yes

In all other JDKs it is enabled by default, and prints correctly yes/no

This PR is fixing this. In jdk17 and up , when manually disabled, the "no" is printed.

todo

• negotiate jira
• In meantime: Is it possible to enable Shenandoah build by default?
• rcap did PR few minutes ahead, his shoudl be used in case of doubts

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