[SPARC64]: Drop %gl to 0 before re-enabling PSTATE_IE in rtrap

If we take a window fault, on SUN4V set %gl to zero before we
turn PSTATE_IE back on in %pstate.  Otherwise if we take an
interrupt we'll end up with corrupt register state.

Signed-off-by: David S. Miller <davem@davemloft.net>