For linux, page 0 is mapped as an execute-only gateway.  A gateway
page is a special bit in the page table that allows a B,GATE insn
within that page to raise processor permissions.  This is how system
calls are implemented for HPPA.

Rather than actually map anything here, or handle permissions at all,
implement the semantics of the actual linux syscall entry points.

Signed-off-by: Richard Henderson <rth@twiddle.net>