xtensa: gdbstub: fix backtracing and stack unwinding

This fixes GDB backtracing by forcibly spilling all registers,
and faking values for WINDOWSTART and WINDOWBASE. This is
effectively telling GDB that only A0-A3 and AR0-AR3 contain
active data and other physical registers do not. GDB then must
rely on spilled values on stack. Otherwise, GDB will try to
look at all AR* registers for previous frame(s). Since we
do not save all AR* register values, there is nothing for GDB
to look at, and thus failing to unwind stack.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>