arch: arm64: irq/switch: accessing nested using _cpu_t

With _kernel_offset_to_nested, we only able to access the nested counter
of the first cpu. Since we are going to support SMP, we need accessing
nested from per cpu.

To get the current cpu, introduce z_arm64_curr_cpu for asm usage,
because arch_curr_cpu could not be compiled in asm code.

Signed-off-by: Peng Fan <peng.fan@nxp.com>