tools/bin: update btfgen to version 0.1

- this version supports type based relocations:
  kinvolk/btfgen#15

- drop poison detection from btfgen.sh: it was bad assumption. sometimes
  the generated relocations are poisoned but they are placed in dead
  branches. it is impossible to detect such cases only by running
  btfgen, so it is better to get rid of the function entirely.