Rollup merge of #39111 - alexcrichton:more-cross-targets, r=brson

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.

cc #38531

diff --cc .travis.yml
index 6cdc4d99f201ebf40eb8d53526ef85ef9e837f47,6732a94e313dbd029f749d03e94abbcacec6532a..f38026857c0e2e509fa11d5511c8540d1e645534
--- 1/.travis.yml
--- 2/.travis.yml
+++ b/.travis.yml
@@@ -13,9 -13,8 +13,9 @@@ osx_image: xcode8.
  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=linux-tested-targets DEPLOY=1
      - env: IMAGE=dist-arm-linux DEPLOY=1
      - env: IMAGE=dist-armv7-aarch64-linux DEPLOY=1
      - env: IMAGE=dist-freebsd DEPLOY=1