feat(riscv): add MOV pseudo-instruction and RVC compressed encoding

Assisted-by: DeepSeek V4 Pro

testdata/csr_riscv64.s

@@ -4,24 +4,23 @@
 #include "textflag.h"
 
 // func readCSR(csr int64) int64
-// Reads a CSR into the return value.
 TEXT ·readCSR(SB), NOSPLIT, $0-16
-	LD a+0(FP), A0
-	CSRRS $0x300, X0, A1
-	SD A1, ret+8(FP)
-	RET
+        MOV a+0(FP), X10
+        CSRRS $0x300, X0, X11
+        MOV X11, ret+8(FP)
+        RET
 
 // func setCSRBit(csr, bit int64) int64
 TEXT ·setCSRBit(SB), NOSPLIT, $0-24
-	LD a+0(FP), A0
-	LD b+8(FP), A1
-	CSRRS $0x304, A1, A2
-	SD A2, ret+16(FP)
-	RET
+        MOV a+0(FP), X10
+        MOV b+8(FP), X11
+        CSRRS $0x304, X11, X12
+        MOV X12, ret+16(FP)
+        RET
 
 // func writeCSR(val int64) int64
 TEXT ·writeCSR(SB), NOSPLIT, $0-16
-	LD a+0(FP), A0
-	CSRRW $0x305, A0, A1
-	SD A1, ret+8(FP)
-	RET
+        MOV a+0(FP), X10
+        CSRRW $0x305, X10, X11
+        MOV X11, ret+8(FP)
+        RET