Changes so that Xen can be compiled with gcc 4.0.2: (by Tristan Gingold)

Changes so that Xen can be compiled with gcc 4.0.2: (by Tristan Gingold)
* functions must be declared before being called.
* a cast or a conditionnal expression is not an lvalue.
* a function cannot be declared static and redeclared extern (or reverse).
* gcc 4.0.2 missed a range optimization (ia64_setreg_unknown_kr).
* ia64_ksyms is not used (removed from Makefile).
* (added by Dan M: since now modified, move gfp.h from linux to linux-xen)