[Local GC] Remove static fields from GC interface (dotnet/coreclr#10566)

[Local GC] Remove static fields from GC interface (dotnet/coreclr#10566)

* Remove max_generation from GC interface

* [Local GC] Clean up the GC interface by removing all static fields
from it:
  1) gcHeapType is replaced by g_heap_type (EE side) and g_gc_heap_type
     (GC side), each of which is set on initialization. g_heap_type is
     read by the DAC to determine what kind of heap is being used.
  2) maxGeneration is not necessary due to the GC DAC changes.

* Rebase against master

* Comments and cleanup

* Use a heap allocation instead of alloca

* Code review feedback: remove a redundant cast and allocate generation count table once, on first use

* Address code review feedback: cache some calls to GetMaxGeneration

Commit migrated from https://github.com/dotnet/coreclr/commit/ed57233f4d26114d45b1293ced80a09f879f72cf