add little script to build libstd

That's easier to use than having to `cd xargo`

.travis.yml

@@ -12,9 +12,7 @@ before_script:
 script:
 - |
   # get ourselves a MIR-ful libstd
-  cd xargo &&
-  RUSTFLAGS='-Zalways-encode-mir' xargo build &&
-  cd ..
+  xargo/build.sh
 - |
   # Test plain miri
   cargo build &&

xargo/build.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+cd "$(readlink -e "$(dirname "$0")")"
+RUSTFLAGS='-Zalways-encode-mir' xargo build