Move TARGET_PHYS_ADDR_SPACE_BITS to target-*/cpu.h.

Removes a set of ifdefs from exec.c.

Introduce TARGET_VIRT_ADDR_SPACE_BITS for all targets other
than Alpha.  This will be used for page_find_alloc, which is
supposed to be using virtual addresses in the first place.

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