Compatibility with generational ZGC

[DanielThomas]

Non-generational Shenandoah and ZGC have two memory managers, one concurrent, one stop-the-world, named <gc> Cycles and <gc> Pauses:

• https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/z/zServiceability.cpp#L123-L124
• https://github.com/openjdk/jdk/blob/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L477-L478

However generational ZGC splits these into major/minor cycles/pauses:

• https://github.com/openjdk/zgc/blob/zgc_generational/src/hotspot/share/gc/z/zServiceability.cpp#L174-L187

Looks like there's no change required for genshen so far, they're using the same memory managers:

• https://github.com/corretto/corretto-17/blob/generational-shenandoah/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L2895-L2909

Allocation Stall is also not a special case, and just another concurrent cycle trigger. It does indicate a allocation stall occurred, but it's not a pause and doesn't tell you how long threads were paused for:

• https://github.com/openjdk/jdk/blob/cd6b54ec40f1d60fbdb6c8aee1e6ba662daca58c/src/hotspot/share/gc/z/zDirector.cpp#L55-L65
• https://github.com/openjdk/jdk/blob/1750a6e2c06960b734f646018fc99b336bd966a5/src/hotspot/share/gc/z/zPageAllocator.cpp#L458-L459
• https://github.com/openjdk/jdk/blob/054c23f484522881a0879176383d970a8de41201/src/hotspot/share/gc/shared/gcCause.cpp#L129-L130

Found while testing genzgc early access builds:

[brharrington]

Looks fine, the only issue is it would alter behavior for some pre jdk17 versions with ZGC, but the data for those was sketchy anyway and those versions are now EOL.