build: allow multiple toolchain defaults

This change enables a fairly commonly-requested use-case, which is to
fall back to the host system's native toolchain when building on AArch64
if the bare-metal toolchain is not available.

In this situation, if the `aarch64-none-elf` GCC toolchain cannot be
located, the build system will look for `aarch64-linux-gnu` before
giving up.

Change-Id: I39d2a8837b651b28cf0eafa92f6003a7f66767a0
Signed-off-by: Chris Kay <chris.kay@arm.com>
5 files changed