gcc-cross: Improve handling of unwind.h

gcc-cross: Improve handling of unwind.h

Rather than building the whole of libgcc to obtain the unwind.h header
file, simply configure it and then install the file. This avoids copying
chunks of data around when we don't need to and building the same thing
twice.

After doing this we need to make sure the target build directory exists
in the libgcc case since it will no longer be created automatically.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>