tools: add etnaviv_gdb plugin

GDB plugin for etnaviv driver debugging. This needs gdb 7.5+ to work.

usage (from gdb):
   source /path/to/etnaviv_gdb.py

Commands:
   gpu-state (prefix|uniforms)
       Show full GPU state (default) or only registers with a certain prefix.
       The special prefix 'uniforms' shows only the shader uniforms.
   gpu-dis
       Disassemble the current shaders.