gdb_server, target: Add target_address_bits()

gdb_server, target: Add target_address_bits()

Targets can use this to expose how many address bits there are.
gdb_server uses this to send gdb the appropriate upper limit in the
memory-map. (Before this change the upper limit would only be correct
for 32-bit targets.)

Change-Id: Idb0933255ed53951fcfb05e040674bcdf19441e1
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4947
Tested-by: jenkins
Reviewed-by: Peter Mamonov <pmamonov@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>