Refactor target specific handling, compile vl.c only once

Move target specific functions and RAM handling to arch_init.c.

Add a flag to QEMUOptions structure to indicate for which
architectures the option is allowed, check the flag
in run time and remove conditional code in option handling.

Now that no target dependencies remain, compile vl.c only once
for all targets.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
