riscv64: support zihintpause disassembly

[newborn22]

This patch supports disassembly for Zihintpause, also adds tests.

The predecessor CL is at https://go-review.googlesource.com/c/arch/+/734081

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Weihong Qiu:

Patch Set 1:

(1 comment)

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Message from Joel Sing:

Patch Set 2: Code-Review+2 Commit-Queue+1

(3 comments)

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Message from Go LUCI:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-01-26T05:37:02Z","revision":"b5536a8d09f13b38e7d4dfe05f35735f513231cc"}

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Message from Joel Sing:

Patch Set 2: -Commit-Queue

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Message from Go LUCI:

Patch Set 2:

This CL has passed the run

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Message from Go LUCI:

Patch Set 2: LUCI-TryBot-Result+1

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