8389126: Serial: Push Full GC roots onto the marking stack

[albertnetymk]

Serial Full GC currently follows roots directly through follow_root and follow_object, while oops found during object scanning are pushed onto the marking stack. Roots and other oops require the same marking behavior, so this cleanup applies mark_and_push_closure to roots as well and drains the stack after each root group. With roots using the same traversal path as other oops, FollowRootClosure and follow_root can be removed. This leaves follow_object with one caller, so its body is moved into follow_stack.

micro:vm.gc.SystemGC is perf-neurtral.

Test: tier1

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• JDK-8389126: Serial: Push Full GC roots onto the marking stack (Enhancement - P4)

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