target-xtensa: add gdb support

Specific xtensa processor overlay for GDB contains register map in
the gdb/xtensa-config.c. This description is used by the GDB to e.g.
parse 'g' response packets and it may be reused in the qemu's gdbstub
(only XTREG definitions for non-pseudoregisters are needed).

Currently mainline GDB does not support operations with privileged SRs
(see http://sourceware.org/ml/gdb/2011-07/msg00075.html). This support
may be enabled, see NUM_CORE_REGS comment in the gdbstub.c

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