support multiple targets

Summary:
Now you may run toolchain_build with the target triplet as an argument.
i.e. toolchain_build arm-eabi or toolchain_build arm-linux-androideabi

Test Plan: run toolchain_build arm-eabi to build arm-eabi

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D527