[IA64] Sanitize assembler code for ia64_sal_os_state

struct ia64_sal_os_state has three semi-independent sections.  The code
in mca_asm.S assumes that these three sections are contiguous, which
makes it very awkward to add new data to this structure.  Remove the
assumption that the sections are contiguous.  Define a macro to shorten
references to offsets in ia64_sal_os_state.

This patch does not change the way that the code behaves.  It just
makes it easier to update the code in future and to add fields to
ia64_sal_os_state when debugging the MCA/INIT handlers.

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>