It turns out a global variable is the only straightforward way to
communicate the kernel's physical load address from where it's known in
initarm() into cpu_mp_start() which is called from non-arm code and
takes no parameters.

This adds the global variable and ensures that all the various copies
of initarm() set it.  It uses the variable in cpu_mp_start(), eliminating
the last uses of KERNPHYSADDR outside of locore.S (where we can now
calculate it instead of relying on the constant).