syscall: fix riscv64 syscall assembly

RISCV has no instruction to store an immediate value to memory, or to
subtract an immediate value. An immediate needs to be moved to a register
first, or in the case of $0, the zero value register (ZERO or R0) can be
used instead. Restore the original riscv-go code so that it compiles.

Updates #27532

Change-Id: I3c60e6f385c59679252a710dbba153357a367516
Reviewed-on: https://go-review.googlesource.com/c/go/+/212761
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/syscall/asm_linux_riscv64.s

@@ -20,14 +20,14 @@ TEXT ·Syscall(SB),NOSPLIT,$0-56
 	BLTU	T0, A0, err
 	MOV	A0, r1+32(FP)	// r1
 	MOV	A1, r2+40(FP)	// r2
-	MOV	$0, err+48(FP)	// errno
+	MOV	ZERO, err+48(FP)	// errno
 	CALL	runtime·exitsyscall(SB)
 	RET
 err:
 	MOV	$-1, T0
 	MOV	T0, r1+32(FP)	// r1
-	MOV	$0, r2+40(FP)	// r2
-	SUB	A0, $0, A0
+	MOV	ZERO, r2+40(FP)	// r2
+	SUB	A0, ZERO, A0
 	MOV	A0, err+48(FP)	// errno
 	CALL	runtime·exitsyscall(SB)
 	RET
@@ -47,14 +47,14 @@ TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	BLTU	T0, A0, err
 	MOV	A0, r1+56(FP)	// r1
 	MOV	A1, r2+64(FP)	// r2
-	MOV	$0, err+72(FP)	// errno
+	MOV	ZERO, err+72(FP)	// errno
 	CALL	runtime·exitsyscall(SB)
 	RET
 err:
 	MOV	$-1, T0
 	MOV	T0, r1+56(FP)	// r1
-	MOV	$0, r2+64(FP)	// r2
-	SUB	A0, $0, A0
+	MOV	ZERO, r2+64(FP)	// r2
+	SUB	A0, ZERO, A0
 	MOV	A0, err+72(FP)	// errno
 	CALL	runtime·exitsyscall(SB)
 	RET
@@ -70,13 +70,13 @@ TEXT ·RawSyscall(SB),NOSPLIT,$0-56
 	BLTU	T0, A0, err
 	MOV	A0, r1+32(FP)	// r1
 	MOV	A1, r2+40(FP)	// r2
-	MOV	$0, err+48(FP)	// errno
+	MOV	ZERO, err+48(FP)	// errno
 	RET
 err:
 	MOV	$-1, T0
 	MOV	T0, r1+32(FP)	// r1
-	MOV	$0, r2+40(FP)	// r2
-	SUB	A0, $0, A0
+	MOV	ZERO, r2+40(FP)	// r2
+	SUB	A0, ZERO, A0
 	MOV	A0, err+48(FP)	// errno
 	RET
 
@@ -94,13 +94,13 @@ TEXT ·RawSyscall6(SB),NOSPLIT,$0-80
 	BLTU	T0, A0, err
 	MOV	A0, r1+56(FP)	// r1
 	MOV	A1, r2+64(FP)	// r2
-	MOV	$0, err+72(FP)	// errno
+	MOV	ZERO, err+72(FP)	// errno
 	RET
 err:
 	MOV	$-1, T0
 	MOV	T0, r1+56(FP)	// r1
-	MOV	$0, r2+64(FP)	// r2
-	SUB	A0, $0, A0
+	MOV	ZERO, r2+64(FP)	// r2
+	SUB	A0, ZERO, A0
 	MOV	A0, err+72(FP)	// errno
 	RET