cmd/internal/obj/arm64,internal/cpu,runtime: add FEAT_MOPS support

[mauri870]

DO NOT REVIEW
DO NOT SUBMIT

FEAT_MOPS (Memory Operations) is an ARM extension introduced as
optional in Armv8.7 and mandatory from Armv8.8 onwards. It provides
CPY{P,M,E} and SET{P,M,E} instruction sequences for hardware-accelerated
memory copy and memory zeroing.

This change adds assembler support for the CPY and SET instruction
families:

CPYP/CPYM/CPYE (Xs), Xn, (Xd) - memory copy
SETP/SETM/SETE Xs, Xn, (Xd) - memory set

It also adds ARM64.HasMOPS detection to internal/cpu via HWCAP2 on
Linux, and fast paths in the runtime for large operations when the
feature is available at runtime: memmove uses CPY{P,M,E} for copies
larger than 256 bytes, and memclrNoHeapPointers uses SET{P,M,E} for
zeroing larger than 512 bytes. The differing thresholds follow clang's
own codegen behaviour for variable-size operations of each kind.

For #79994

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Mauri de Souza Meneguzzo:

Patch Set 2:

(1 comment)

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