build-support: Use equivalent valid exit code

`exit -1` is equivalent to `exit 255`, since Bash does modulo 256 on the
number.

As per ShellCheck:

> SC2242 (error): Can only exit with status 0-255. Other data should be
> written to stdout/stderr.

pkgs/build-support/bintools-wrapper/macos-sierra-reexport-hack.bash

@@ -136,7 +136,7 @@ while (( "${#norm[@]}" )); do
                 norm=("${norm[@]:1}")
             else
                 echo "ld-wrapper: Internal Error: Invalid normalized argument" >&2
-                exit -1
+                exit 255
             fi
             ;;
     esac