sim: common: add align_{up,down} to match gdb

We have ALIGN_{8,16,PAGE} and FLOOR_PAGE macros (where PAGE is defined as
4k) which were imported from the ppc sim.  But no other sim utilizes these
and hardcoding the sizes in the name is a bit limiting.

Let's delete these and import the two general macros that gdb uses:
	align_up(addr, bytes)
	align_down(addr, bytes)

This in turn allows us to cut over the Blackfin code immediately.