runtime: rename atoi to _atoi on Plan 9

Following change 2154, the goatoi function
was renamed atoi.

However, this definition conflicts with the
atoi function defined in the Plan 9 runtime,
which takes a []byte instead of a string.

This change fixes the build on Plan 9.

Change-Id: Ia0f7ca2f965bd5e3cce3177bba9c806f64db05eb
Reviewed-on: https://go-review.googlesource.com/2165Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/os1_plan9.go

@@ -70,7 +70,7 @@ func getpid() uint64 {
 	for c[0] == ' ' || c[0] == '\t' {
 		c = c[1:]
 	}
-	return uint64(atoi(c))
+	return uint64(_atoi(c))
 }
 
 func osinit() {
@@ -254,7 +254,7 @@ func badsignal2() {
 	exits(&_badsignal[0])
 }
 
-func atoi(b []byte) int {
+func _atoi(b []byte) int {
 	n := 0
 	for len(b) > 0 && '0' <= b[0] && b[0] <= '9' {
 		n = n*10 + int(b[0]) - '0'