The current implementation for testing generational ZGC with jtreg is implemented with a filter on the mode flag `ZGenerational`. Because of this only environments which set this flag explicitly will run most of the tests.
So they get missed in Github Actions and for developers running jtreg locally without supplying the `ZGenerational` flag.

The proposed change here is to introduce two new jtreg requirement properties, `vm.gc.ZGenerational` and `vm.gc.ZSinglegen`. These flags will effectively behave the same as the existing `vm.gc.<GC>` flags but also take the specific ZGC mode in account.

If no gc flags are supplied to jtreg and the `vm.gc.Z` is true (the build includes ZGC) both `vm.gc.ZGenerational` and `vm.gc.ZSinglegen` will be true.

If `-XX:+UseZGC` is supplied then both `vm.gc.ZGenerational` and `vm.gc.ZSinglegen` will also be true.

If `-XX:{+,-}ZGenerational` is supplied then either `vm.gc.ZGenerational` or `vm.gc.ZSinglegen` be true depending on the flags value.

And if `vm.gc.Z` is false both `vm.gc.ZGenerational` and `vm.gc.ZSinglegen` will be false.

This change also splits the relevant tests into two distinct runs for the two modes. And the respective test ids are set to `ZGenerational` or `ZSinglegen` to make it easier to distinguish the runs.

This also solves the issue that some compiler tests will never run with generational ZGC unless the `TEST_VM_FLAGLESS` is set. This is because the current filter `vm.opt.final.ZGenerational` requires the flag to be explicit, but these compiler tests uses `vm.flagless`.

The introduction of `vm.gc.ZGenerational` and `vm.gc.ZSinglegen` harmonizes the way you specify generational / single gen ZGC test with the way it is done for other gcs with `vm.gc.<GC>`