xtensa: add support for the Xtensa architecture

The Xtensa architecture had been removed because it required special
handling and depended on additional directories and files that became
obsolete over time. This change is more aligned to other architectures.

[Thomas: rebased on top of the "arch: improve definition of gcc mtune,
mcpu, etc." patch].

Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>