gcc-target: Limit compile to host targets, don't build runtimes.
Currently the gcc builds are building copies of the target libraries
that we never use (it isn't installed in do_install). This is a rather
pointless waste of cpu time.
Instead just compile the host targets. Comparing the package output of
this compared to a previous build shows that the unwind.h header is
missing since its provided by gcc. Fix this simply by copying it in.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
