Remove the Xtensa architecture

As stated in commit 555c2585, the
Xtensa architecture has been introduced in 2009 and never changed
since its initial introduction. It requires some special handling that
is a bit annoying, and despite our call to the initial developers, and
the announcement of the deprecation of the architecture during the
2012.05, nothing has happened. Therefore, drop support for this
architecture.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: me
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>