8334247: [PPC64] Consider trap based nmethod entry barriers

[TheRealMDoerr]

We can shrink nmethod entry barriers to 4 instructions (from 8) using conditional trap instructions. Some benchmarks seem to show very small improvements. At least the code size reduction is an advantage.

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• JDK-8334247: [PPC64] Consider trap based nmethod entry barriers (Enhancement - P4)

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