toolchain: make external toolchain the default for AArch64

Our internal toolchain backend does not yet have support for AArch64,
and Crosstool-NG also does not have support for AArch64 at the moment
(though it should be coming quickly since the Linaro AArch64 toolchain
is generated with a modified Crosstool-NG version).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>