docker: Install toolchains on arm64 host

Refactor the code to support downloading toolchains for arm64 as well as
x86_64

There doesn't seem to be an xtensa toolchain for arm64 at the same
location, so download that only on x86

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
1 file changed