Auto merge of #2510 - alexcrichton:xcompile, r=brson

Prepare for ARM/FreeBSD/NetBSD nightlies

This commit beefs up Cargo's makefiles to support nightly builds of Cargo for
multiple platforms. This primarily involves vendoring the logic of how to build
OpenSSL for statically linking against Cargo into the Makefiles directly. We'll
have to update the version of OpenSSL as releases are made, but we essentially
already do that with the normal docker container.

The Linux nightlies will still run in the normal dist docker container (a really
old CentOS build) and builds for new platforms will happen in the standard
linux-cross container we use for other cross builds. The nightly versions of
these will produce Cargo tarballs for a whole bunch of platforms to get
uploaded.

This has been tested in the `alexcrichton/rust-slave-linux-cross:2016-03-17b`
docker container for the 3 ARM targets and FreeBSD target. NetBSD will come once
rust-lang/rust#32407 lands.