gdb: Port gdb 7.7.1 recipes from oe-core

* Port the gdb 7.7.1 recipes from oe-core, this is due to issues with
  re-producing a functional gdb for 7.8.1+
* Default MicroBlaze machines to use gdb 7.7.1

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>