toolchain-external: add publicly available musl toolchains

This commit adds ten toolchains based on the musl C library that are
publicly available from the musl-cross project.

[Peter: fix ppc prefix, only for classic ABI]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>