xen: define __section() and friends and use them for section annotations.

xen: define __section() and friends and use them for section annotations.

By itself this is just code-tidying, but it's also useful for the
following patch, which will adjust __section() for clang compiles.

Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Tim Deegan <tim@xen.org>