Add support for building other architectures.

Add support for building other architectures.

Since we often make changes that might break on other architectures,
let the buildbot deal with some of these for us. They can be invoked
by `bionicbb:ARCH`, where `ARCH` is one of:

 * arm
 * aarch64
 * mips
 * mips64
 * x86
 * x86_64

Specifying arm isn't particularly interesting (since the default
target for the buildbot is hammerhead), but there are some differences
in the math instructions available for the default ARM target, so it
could be helpful for testing changes to the compiler-rt builtins.

Change-Id: I94018fd3c30d26fcf405e747fc633cbdd08ff4e5