cmd/internal/obj/riscv: add support for Zihintpause extension on riscv64

[lxq015]

This patch adds support for the Zihintpause extension to the riscv64 architecture. This extension is a mandatory instruction set extension in the RVA23 architecture profile (RVA23 profile), and its PAUSE instruction can reduce energy consumption while executing spin-wait code sequences.

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Patch Set 1:

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This PR (HEAD: f504fdd) has been imported to Gerrit for code review.

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