toolchain-external: restrict Musl external toolchain to Microblaze BE

As reported by William Welch <bvwelch@gmail.com>, the Musl external
toolchain provided by the musl-cross project is only Microblaze
big-endian. In fact, Musl seems to only support the big endian variant
of the Microblaze architecture, with the microblaze-* tuple.

This commit makes sure the Musl pre-built external toolchain provided
by musl-cross can only be selected for a big-endian Microblaze
configuration.

Reported-by: William Welch <bvwelch@gmail.com>
Cc: William Welch <bvwelch@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>