xtensa: implement call0 ABI support in assembly

Replace hardcoded register and opcode names with ABI-agnostic macros.
Add register save/restore code where necessary. Conditionalize windowed
only or call0 only code. Add stack initialization matching _switch_to
epilogue to copy_thread.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>