x/arch/arm64/arm64asm: add FEAT_MOPS instruction decoding

[mauri870]

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FEAT_MOPS (Memory Operations) is an ARM extension introduced as optional in Armv8.7 and mandatory from Armv8.8 onwards. It provides three-instruction sequences for hardware-accelerated memory copy and memory set:

CPYP/CPYM/CPYE [<Xd>]!, [<Xs>]!, <Xn>! - memory copy
SETP/SETM/SETE [<Xd>]!, <Xn>!, <Xs> - memory set

This change adds decoder support for all six instructions. The operand forms are unique to MOPS: the destination pointer is a memory write-back ([Xd]!), and the count and source registers carry explicit write-back notation (Xn!) to reflect that the CPU updates them in-place during execution. Two new Arg types are introduced to represent these forms: MemMOPS for [Xd]! and RegWriteBack for Xn!.

The per-instruction mask is 0xffe0fc00, which covers the op1[23:22] field that distinguishes the prologue, main, and epilogue variants of each family.

For golang/go#79994

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Message from Gopher Robot:

Patch Set 1:

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