toolchain-external: mark Synopsys toolchain as broken

toolchain-external: mark Synopsys toolchain as broken

This uClibc toolchain does not provide an appropriate uClibc
configuration for Buildroot: missing IPv6, missing nsl stub, missing
program invocation, etc. Therefore, we mark it as broken, waiting for
a new upstream release of a new toolchain.

We keep around the toolchain-external Synopsys code anyway, since it
will most likely be identical for the new toolchain version. However,
we remove all the quirks that were introduced to start work around
issues related to this toolchain.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>