travis: Expand the cross linux image

This expands the `cross` travis matrix entry with a few more targets that our
nightlies are building:

* x86_64-rumprun-netbsd
* arm-unknown-linux-musleabi
* arm-unknown-linux-musleabihf
* armv7-unknown-linux-musleabihf
* mips-unknown-linux-musl
* mipsel-unknown-linux-musl

This commit doesn't compile custom toolchains like our current cross-image does,
but instead compiles musl manually and then compiles libunwind manually (like
x86_64) for use for the ARM targets and just uses openwrt toolchains for the
mips targets.

.travis.yml

@@ -13,7 +13,7 @@ osx_image: xcode8.2
 matrix:
   include:
     # Linux builders, all docker images
-    - env: IMAGE=arm-android DEPLOY=1
+    - env: IMAGE=android DEPLOY=1
     - env: IMAGE=cross DEPLOY=1
     - env: IMAGE=dist-arm-linux DEPLOY=1
     - env: IMAGE=dist-armv7-aarch64-linux DEPLOY=1