runtime: fix typos in comments

Change-Id: Id169b68cc93bb6eb4cdca384efaaf971fcfa32b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/666316
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: sophie-zhao

src/runtime/mkduff.go

@@ -179,8 +179,8 @@ func copyARM64(w io.Writer) {
 
 func zeroLOONG64(w io.Writer) {
 	// R0: always zero
-	// R19 (aka REGRT1): ptr to memory to be zeroed
-	// On return, R19 points to the last zeroed dword.
+	// R20: ptr to memory to be zeroed
+	// On return, R20 points to the last zeroed dword.
 	fmt.Fprintln(w, "TEXT runtime·duffzero<ABIInternal>(SB), NOSPLIT|NOFRAME, $0-0")
 	for i := 0; i < 128; i++ {
 		fmt.Fprintln(w, "\tMOVV\tR0, (R20)")