MicroBlaze: Add microblaze_get_next_pcs

This patch enables software single stepping for gdbserver target

* gdb/microblaze-tdep.c: Add microblaze_get_next_pcs

Signed-off-by: David Holsgrove <david.holsgrove@petalogix.com>
Signed-off-by: Nathan Rossi <nathan.rossi@petalogix.com>
Signed-off-by: Mahesh Bodapati <mbodapat@xilinx.com>
Signed-off-by: Gopi Kumar Bulusu <gopi@sankhya.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>