xtensa: gdbstub: fix compilation

Building gdbstub for xtensa is failing currently with multiple
failures like

arch/xtensa/core/gdbstub.c:432:24: error: invalid operands to \
binary - (have 'int *' and 'const struct arch_esf *')
  432 |         if ((int *)bsa - stack > 4) {

Fix them by using appropriate pointer types.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>