feat(arm64): encode pairs, atomics, crypto, system and NEON slices

Assisted-by: DeepSeek V4.1 Flash

testdata/verify/integer_arm64.s

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+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Differential kernel for the arm64 integer slice: carry-setting arithmetic,
+// widening multiplies, bit manipulation, conditional compares, the compare
+// and test branches, ADR and the wide-constant moves.  Every function is
+// byte-compared against go tool asm.
+
+#include "textflag.h"
+
+// func carryArith()
+TEXT ·carryArith(SB), NOSPLIT, $0-0
+	ADC  R0, R2, R12
+	ADCS R23, R22, R22
+	ADC  $0, R1
+	SBC  R25, R10, R26
+	SBCS R5, R9, R5
+	SBCS $0, R1
+	RET
+
+// func wideningMul()
+TEXT ·wideningMul(SB), NOSPLIT, $0-0
+	MUL   R4, R3, R0
+	MSUB  R19, R16, R26, R2
+	SMULH R24, R20, R24
+	UMULH R24, R20, R24
+	RET
+
+// func bitManip()
+TEXT ·bitManip(SB), NOSPLIT, $0-0
+	RBIT  R11, R4
+	REV   R1, R2
+	CLZ   R21, R9
+	REVW  R1, R2
+	CLSW  R1, R2
+	UBFX  $33, R17, $25, R5
+	UBFXW $4, R1, $9, R2
+	RET
+
+// func condCompare()
+TEXT ·condCompare(SB), NOSPLIT, $0-0
+	CCMP  LE, R7, $19, $3
+	CCMP  LT, R30, R6, $7
+	CCMN  EQ, R1, R2, $3
+	CCMPW LE, R7, $19, $3
+	RET
+
+// func branchForms()
+TEXT ·branchForms(SB), NOSPLIT, $0-0
+	CBZ   R1, target
+	CBNZ  R7, target
+	CBNZW R2, target
+	TBZ   $4, R7, target
+	TBNZ  $33, R7, target
+	ADR   target, R10
+
+target:
+	RET
+
+// func wideMoves()
+TEXT ·wideMoves(SB), NOSPLIT, $0-0
+	MOVK  $1234, R5
+	MOVK  $305397760, R5
+	MOVKW $1234, R5
+	MOVK  $16771847290880, R21
+	RET