arch/riscv: add Zihintpause support

arch/riscv: add Zihintpause support

Implement support for the ZiHintPause extension.

The PAUSE instruction is a HINT that indicates the current hart’s rate
of instruction retirement should be temporarily reduced or paused.

Reviewed-by: Heiko Stuebner <[email protected]>
Tested-by: Heiko Stuebner <[email protected]>
Signed-off-by: Dao Lu <[email protected]>
[Palmer: Some minor merge conflicts.]
Link: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Palmer Dabbelt <[email protected]>