toolchain-buildroot: only allow the selection of Musl for Microblaze BE

The Musl C library only supports Microblaze BE, not Microblaze LE, so
this commit adjusts the dependencies of the toolchain-buildroot
package to not allow the selection of Musl on Microblaze LE.

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