toolchain-external: bump musl external toolchains to 1.1.1

And while we're at it, factorize the definition of the musl version,
since it's common to the definition of the tarball names for the
various supported architectures.

[Peter: Adjust Config.in info to match new version]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>