feat(riscv64,loong64): encode AMO atomics, vector slices and bit ops

Assisted-by: GLM 5.3 Flash

testdata/verify/atomics_loong64.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 loong64 atomics: the AM* family in its plain
+// and _dbar (acquire/release) forms, spelled as the runtime's
+// atomic_loong64.s spells them.  Every AM* takes three operands:
+// value, (address), result.
+
+#include "textflag.h"
+
+TEXT ·plain(SB), NOSPLIT, $0-0
+	AMSWAPB R14, (R13), R12
+	AMSWAPH R14, (R13), R12
+	AMSWAPW R5, (R4), R6
+	AMSWAPV R5, (R4), R0
+	AMCASB  R14, (R13), R12
+	AMCASH  R6, (R4), R5
+	AMCASW  R6, (R4), R5
+	AMCASV  R6, (R4), R5
+	AMADDW  R5, (R4), R0
+	AMADDV  R14, (R13), R12
+	AMANDW  R5, (R4), R6
+	AMANDV  R5, (R4), R6
+	AMORW   R5, (R4), R0
+	AMORV   R5, (R4), R6
+	AMXORW  R5, (R4), R6
+	AMXORV  R5, (R4), R6
+	AMMAXW  R5, (R4), R6
+	AMMAXV  R5, (R4), R6
+	AMMINW  R5, (R4), R6
+	AMMINV  R5, (R4), R6
+	AMMAXWU R5, (R4), R6
+	AMMAXVU R5, (R4), R6
+	AMMINWU R5, (R4), R6
+	AMMINVU R5, (R4), R6
+	RET
+
+TEXT ·dbar(SB), NOSPLIT, $0-0
+	AMADDDBW  R5, (R4), R6
+	AMADDDBV  R5, (R4), R6
+	AMANDDBW  R5, (R6), R0
+	AMANDDBV  R5, (R4), R6
+	AMORDBW   R5, (R6), R0
+	AMORDBV   R5, (R4), R6
+	AMSWAPDBW R5, (R4), R6
+	AMSWAPDBV R5, (R4), R0
+	AMCASDBW  R6, (R4), R5
+	AMCASDBV  R6, (R4), R5
+	RET