math/big: fix loong64 assembly for vet

Vet is failing on this code because some arguments of mulAddVWW
got renamed in the go decl (CL 664895) but not the assembly accessors.

Looks like the assembly got written before that CL but checked in
after that CL.

Change-Id: I270e8db5f8327aa2029c21a126fab1231a3506a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/665717
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

Author: randall77

src/math/big/arith_loong64.s

@@ -98,19 +98,19 @@ TEXT ·lshVU(SB),NOSPLIT,$0
 TEXT ·rshVU(SB),NOSPLIT,$0
 	JMP ·rshVU_g(SB)
 
-// func mulAddVWW(z, x []Word, y, r Word) (c Word)
+// func mulAddVWW(z, x []Word, m, a Word) (c Word)
 TEXT ·mulAddVWW(SB),NOSPLIT,$0
 	// input:
 	//   R4: z
 	//   R5: z_len
 	//   R7: x
-	//   R10: y
-	//   R11: r
+	//   R10: m
+	//   R11: a
 	MOVV	z+0(FP), R4
 	MOVV	z_len+8(FP), R5
 	MOVV	x+24(FP), R7
-	MOVV	y+48(FP), R10
-	MOVV	r+56(FP), R11
+	MOVV	m+48(FP), R10
+	MOVV	a+56(FP), R11
 	SLLV	$3, R5
 	MOVV	$0, R6
 loop: