syscall: add missing type conversion

The gofrontend incorrectly accepted code that was missing a type conversion.
The test case for this is bug518.go in https://go.dev/cl/536537.
Future CLs in this series will detect the type error.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/536638

gcc/go/gofrontend/MERGE

-c201fa2a684ada551ca9a0825a3075a0a69498de
+081ec9824a74ec9d82628d8d2f6b9a7a4c35a529
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/syscall/errstr.go

@@ -24,7 +24,7 @@ func Errstr(errnum int) string {
 			}
 			return string(b[:i])
 		}
-		if errno != ERANGE {
+		if Errno(errno) != ERANGE {
 			return "strerror_r failure"
 		}
 	}