RFR: 8294729: [s390] Implement nmethod entry barriers

[Tyler Steele]

On Mon, 10 Oct 2022 09:08:19 GMT, Lutz Schmidt <lucy at openjdk.org> wrote:

>> The branch target is constant once the stub is created. We don't need to load it from memory.
>> I meant `__ z_larl(Z_R1_scratch, StubRoutines::zarch::nmethod_entry_barrier());`
>
> Good idea! One instruction instead of two, plus avoiding a data interlock when loading the constant address.

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.

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