Add multicore simulator

* build-epiphany-sdk.sh: Download and build Epiphany multicore simulator
by default.
* build-toolchain.sh: Support building and installing the multicore
simulator so that it doesn't conflict with GDB from the toolchain.
* component-versions: Track the Epiphany multicore simulator branch.
* download-toolchain.sh: Add option to download multi-core simulator.

Conflicts:
	ChangeLog
	build-toolchain.sh