ext-toolchain: Automatically set -m64 if the architecture is x86_64

Without the -m64, choosing the x86_64 architecture with a Sourcery external
toolchain will result in a 32-bit rootfs.

Also simplified the help text to reflect this change.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>