cmd/internal/obj/riscv: implement RV64I integer computational instructions

Add support for assembling RV64I integer computational instructions.

Based on the riscv-go port.

Updates #27532

Integer Computational Instructions (RV64I)

Change-Id: I1a082b3901c997da309d737d081f57ea2821bc62
Reviewed-on: https://go-review.googlesource.com/c/go/+/196838Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/internal/asm/testdata/riscvenc.s

@@ -95,6 +95,17 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	SB	$0, X5, X6				// 23005300
 	SB	$4, X5, X6				// 23025300
 
+        // 5.2: Integer Computational Instructions (RV64I)
+	ADDIW	$1, X5, X6				// 1b831200
+	SLLIW	$1, X5, X6				// 1b931200
+	SRLIW	$1, X5, X6				// 1bd31200
+	SRAIW	$1, X5, X6				// 1bd31240
+	ADDW	X5, X6, X7				// bb035300
+	SLLW	X5, X6, X7				// bb135300
+	SRLW	X5, X6, X7				// bb535300
+	SUBW	X5, X6, X7				// bb035340
+	SRAW	X5, X6, X7				// bb535340
+
 	// 5.3: Load and Store Instructions (RV64I)
 	LD	$0, X5, X6				// 03b30200
 	LD	$4, X5, X6				// 03b34200

src/cmd/internal/obj/riscv/obj.go

@@ -401,6 +401,17 @@ var encodingForAs = [ALAST & obj.AMask]encoding{
 	ASH & obj.AMask:  sIEncoding,
 	ASB & obj.AMask:  sIEncoding,
 
+	// 5.2: Integer Computational Instructions (RV64I)
+	AADDIW & obj.AMask: iIEncoding,
+	ASLLIW & obj.AMask: iIEncoding,
+	ASRLIW & obj.AMask: iIEncoding,
+	ASRAIW & obj.AMask: iIEncoding,
+	AADDW & obj.AMask:  rIIIEncoding,
+	ASLLW & obj.AMask:  rIIIEncoding,
+	ASRLW & obj.AMask:  rIIIEncoding,
+	ASUBW & obj.AMask:  rIIIEncoding,
+	ASRAW & obj.AMask:  rIIIEncoding,
+
 	// 5.3: Load and Store Instructions (RV64I)
 	ALD & obj.AMask: iIEncoding,
 	ASD & obj.AMask: sIEncoding,