target-alpha: Implement call_pal without an exception

The destination of the call_pal, and the cpu state, is very predictable;
there's no need for exiting the cpu loop.

Signed-off-by: Richard Henderson <rth@twiddle.net>
target-alpha: Use goto_tb in call_pal

With appropriate flushing when the PALBR changes, the target of
a CALL_PAL is so predictable we can chain to it.

Signed-off-by: Richard Henderson <rth@twiddle.net>
target-alpha: Consider the superpage when threading and ending TBs

This allows significantly more threading, and occasionally larger TBs,
when processing code for the kernel and PALcode.

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