8383807: Parallel: Improve Young GC stack trimming

[albertnetymk]

Improve Parallel GC promotion stack trimming for the single-GC-worker case.

Previously, PSPromotionManager used a drain target of 0 when ParallelGCThreads == 1, so intermediate stack-processing points fully drained the promotion stack. Full draining is expensive and unnecessary there; the stack only needs bounded trimming, with explicit full drains kept at termination points.

This change separates bounded trimming from full draining:

• trim_stacks() trims to GCDrainStackTargetSize
• drain_stacks() fully drains the stack

Running the attachment in JBS shows ~8% improvement for the -XX:ParallelGCThreads=1 case, and perf-neutral for multiple gc threads case.

Test: tier1

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Issue

• JDK-8383807: Parallel: Improve Young GC stack trimming (Enhancement - P4)

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