MicroBlaze: Update gdb/NEWS for GNU/Linux gdbserver target

* gdb/NEWS : Add MicroBlaze gdbserver target

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: Michael J. Eager <eager@eagercon.com>