RFR: 8294729: [s390] Implement nmethod entry barriers

[Tyler Steele]

On Tue, 11 Oct 2022 15:49:11 GMT, Lutz Schmidt <lucy at openjdk.org> wrote:

>> I made this change. Though I do want to confirm that `__ z_larl(Z_R1_scratch, StubRoutines::zarch::nmethod_entry_barrier());` is the correct version, and not `&StubRoutines::zarch::nmethod_entry_barrier()` which is also mentioned above. Since nmethod_entry_barrier returns an `address` (and address is already a `char*`) I don't believe we want the ampersand.
>
> You are right. We don't want the '&'. I first assumed (disregarding the parentheses) this would be a field where the address of a generated stub is stored.

I think I had the same thought when I wrote the code initially, so I was doubting my reasoning. Thanks for confirming :-)

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PR: https://git.openjdk.org/jdk/pull/10558