common: Fix bash shell script invocation

`#!/usr/bin/env bash` should be used to avoid hard paths.

To address:

  dockcross-manylinux-x64: bad interpreter: /bin/bash^M: no such file or directory

linux-x86/i686-linux-gnu-as.sh

@@ -1,2 +1,2 @@
-#!/bin/bash
+#!/usr/bin/env bash
 exec ${0/${CROSS_TRIPLE}-/x86_64-linux-gnu-} --32 "$@"

linux-x86/i686-linux-gnu.sh

@@ -1,2 +1,2 @@
-#!/bin/bash
+#!/usr/bin/env bash
 exec ${0/*${TOOLCHAIN}-/\/usr\/bin\/x86_64-linux-gnu-} -m32 "$@"