xtensa: gdbstub: fix unavailable register reporting

To report unavailable registers to the GDB client, the GDB stub has
to return a string like "xx" in the value string.

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