kernel: Move per-cpu _kernel_t fields into separate struct

When in SMP mode, the nested/irq_stack/current fields are specific to
the current CPU and not to the kernel as a whole, so we need an array
of these.  Place them in a _cpu_t struct and implement a
_arch_curr_cpu() function to retrieve the pointer.

When not in SMP mode, the first CPU's fields are defined as a unioned
with the first _cpu_t record.  This permits compatibility with legacy
assembly on other platforms.  Long term, all users, including
uniprocessor architectures, should be updated to use the new scheme.

Fundamentally this is just renaming: the structure layout and runtime
code do not change on any existing platforms and won't until someone
defines a second CPU.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>