Android cross-compile

r? @graydon

This is the bulk of the makefile work necessary to support cross-compiling with multiple toolchains. With this we can cross-compile to Android, but not run the test suite on Android. There is still some polish yet to add, but I want to get this merged.

Key changes:

* The build now thinks about three kinds of triples: the *build* triple, which is the architecture the rust build takes place on, the *host* triples, which are architectures for which the rust build produces rust compliers, and the *target* triples, which are architectures for which the produced compilers can compile code. The host triples contain the build triple, and the target triples are a superset of the host triples.
* `platform.mk` produces distinct toolchain variables (CC, CXX, etc.) for each target triple, and in turn most of the makefiles are now parametrized over target triples.
* `configure` requires `--android-ndk-path` when `arm-unknown-android` is a target triple

As an example, to cross-compile to android, you configure with `../configure --target-triples=arm-unknown-android --android-ndk-path=~/dev/android-standalone-ndk-14`. In this case the build triple and the host triples are automatically inferred, and the build triple added to the target triples, so if you are running on x86_64-unknown-linux-gnu, build-triple=`x86_64-unknown-linux-gnu`, host-triples=`x86_64-unknown-linux-gnu` and target-triples=`x86_64-unknown-linux-gnu,arm-unknown-android`.

Thanks to @yichoi who did most of this work.

Issue #4513.