add 'cpu' argument for scons

Instead of using -mcpu=cortex-a15 on the .S files blindly, we should be
consistent about what CPU we're targeting. I'm adding a 'cpu' argument which
maps to '-mcpu' at the compiler level. This also requires determining whether
the target architecture supports the udiv/sdiv instructions via macros.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
test6zr: move incorrectly placed r4 decrement

This corrects a regression introduced by 2af1db78a.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
test6zr: only initialize r9 once

Signed-off-by: Steven Noonan <steven@uplinklabs.net>