RFR: 8256732: Zero: broken +ZeroTLAB exposes badly initialized memory

[Aleksey Shipilev]

On Tue, 24 Nov 2020 01:40:37 GMT, David Holmes <dholmes at openjdk.org> wrote:

> If ZeroTLAB is not actually zeroing all memory returned via TLAB then something seems far more broken than just Zero!

I think I figured it out: Zero picks the short stick with space mangling. The rest of Hotspot code does not call that method directly, and instead goes through various `MemoryAllocator` classes (which I am not sure we can touch without the VM transition). So, I fixed this by allowing debug builds to initialize object field block again.

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PR: https://git.openjdk.java.net/jdk/pull/1343