internal/runtime/atomic: add Xchg8 for 386

[mauri870]

For #69735

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[gopherbot]

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 Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-10-17T13:24:31Z","revision":"b7fa76e43ee930baca7a74021672a0714b8f8d57"}

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Message from Mauri de Souza Meneguzzo:

Patch Set 1: Run-TryBot+1

(1 comment)

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Gopher Robot:

Patch Set 1: TryBot-Result+1

(1 comment)

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Message from Mauri de Souza Meneguzzo:

Patch Set 1: -Commit-Queue

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Message from Go LUCI:

Patch Set 1:

This CL has passed the run

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Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result+1

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Message from Keith Randall:

Patch Set 1: Auto-Submit+1 Code-Review+2

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Message from Keith Randall:

Patch Set 1: Code-Review+1

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This PR is being closed because golang.org/cl/620756 has been merged.