feat(asm): encode the arm64 FP immediate moves

Assisted-by: GLM 5.3 Flash

testdata/verify/fpimm_arm64.s

@@ -0,0 +1,34 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Differential kernel for the arm64 FP immediate moves: the FMOV (immediate)
+// instruction for the 8-bit encodable values and the FMOV-from-ZR move for
+// zero.  The values outside the 8-bit field pool through the toolchain's
+// $f64 symbols with a PC-relative relocation; gasm carries no such pool and
+// rejects the spelling instead (its previous encoding was wrong code).
+// Every function is byte-compared against go tool asm.
+
+#include "textflag.h"
+
+// func encodable()
+TEXT ·encodable(SB), NOSPLIT, $0-0
+	FMOVS $ (4.0), F0
+	FMOVD $ (4.0), F0
+	FMOVS $ (0.265625), F1
+	FMOVD $ (0.1796875), F2
+	FMOVS $ (0.96875), F3
+	FMOVD $ (28.0), F4
+	FMOVD $ (-4.0), F6
+	FMOVD $ (1.0), F7
+	FMOVD $ (0.5), F8
+	RET
+
+// func zeroes()
+TEXT ·zeroes(SB), NOSPLIT, $0-0
+	FMOVD $0, F0
+	FMOVS $0, F1
+	FMOVD $ (0.0), F5
+	FMOVS $ (0.0), F6
+	FMOVD ZR, F2
+	FMOVS ZR, F3
+	RET