util/xcompile/xcompile: use env(1) to find bash

Not all systems put bash at /bin/bash.

Change-Id: Ib58cd2f6cf330b5b2678d55bb929696872fba9c9
Signed-off-by: Jonathan A. Kollasch <jakllsch@kollasch.net>
Reviewed-on: http://review.coreboot.org/10808
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/xcompile/xcompile

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # This file is part of the coreboot project.
 #