toolchain-external: update ARMeb musl dependency

toolchain-external: update ARMeb musl dependency

ARM big-endian is different from ARMv4/5/6 and ARMv7. Big-endian on
ARMv4/5/6 is BE-32 while big-endian on ARMv7 is BE-8, which are not
compatible.

Therefore, the musl big endian toolchain that is built for ARMv4
cannot work for ARMv7, it can only work for ARMv4/5/6.

This commit updates the musl toolchain dependency accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>