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.

Change-Id: I1da941f97e32695b809f504ad448b1e4b66843e3
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/536638
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

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"
 		}
 	}