toolchain-external: add dependency on BR2_HOST_ARCH

The pre-build external toolchains are all built for x86, so they are
only available if the build machine is a x86 or x86-64 machine.

[Peter: microblaze toolchains are 64bit]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>