xen: remove more declarations from C files.

xen: remove more declarations from C files.

This patch moves some more, mostly data, extern declarations into
header files.   I haven't been as strict as I was with functions;
in particular there are a number of declarations of assembler labels
that are only used in one place.  I've also left a few compat-mode
tricks, and all the magic in symbols.c

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>