arch/xtensa: soc/intel_adsp: Rework MP code entry

Instead of passing the crt1 _start function as the entry code for
auxiliary CPUs, use a tiny assembly stub instead which can avoid the
runtime testing needed to skip the work in _start.  All the crt1 code
was doing was clearing BSS (which must not happen on a second CPU) and
setting the stack pointer (which is wrong on the second CPU).

This allows us to clean out the SMP code in crt1.

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