Don't include <asm/ia64regs.h> unnecessarily.

Don't include <asm/ia64regs.h> unnecessarily.

We only need that header when compiling with icc, since the gcc variant of
ia64_get_bsp() uses in-line assembly code.  Per report from Frank Brendel,
the header doesn't exist on all IA64 platforms; so don't include it unless
we need it.