runtime: remove stackcheck from openbsd/riscv64

[mauri870]

CL 541735 added runtime·stackcheck to mstart_stub on openbsd/riscv64,
but on OpenBSD (and any other platform backed by OS-allocated stacks)
mp.g0.stack.hi and mp.g0.stack.lo are both zero at that point. As a
result the port is currently broken.

No other OpenBSD architecture currently calls stackcheck.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Mauri de Souza Meneguzzo:

Patch Set 1: Commit-Queue+1

(1 comment)

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-07-23T19:16:31Z","revision":"49a30a793d59270ae400eb84572e60976d50f4a1"}

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Message from Mauri de Souza Meneguzzo:

Patch Set 1: -Commit-Queue

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Patch Set 1:

This CL has passed the run

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Patch Set 1: LUCI-TryBot-Result+1

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Message from Joel Sing:

Patch Set 1: Code-Review+2

(2 comments)

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Message from Keith Randall:

Patch Set 1: Code-Review+1

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