add -lgcc flag to fix "undefined reference to __aeabi_llsr"

[adhintz]

fixes commaai/openpilot#522

The sizes look fine:

make clean && make recover; ls -l obj/bootstub.panda.*

with this pull request (-Os and -lgcc)
   8024 Feb  5 21:57 obj/bootstub.panda.bin
 182460 Feb  5 21:57 obj/bootstub.panda.elf
  60272 Feb  5 21:57 obj/bootstub.panda.o

original before the -Os commit (-O2 and no -lgcc)
   8988 Feb  5 22:00 obj/bootstub.panda.bin
 187596 Feb  5 22:00 obj/bootstub.panda.elf
  68400 Feb  5 22:00 obj/bootstub.panda.o

However I'm not sure how to test bootstub beyond using it to flash a Panda and seeing if the Panda works.

Saw this fix suggested here: https://stackoverflow.com/questions/23410221/linking-libgcc-into-a-nostdlib-compilation

[rbiasini]

FYI, I've reverted -Os to -O2 for now. If this fixes the issue I'll put -Os back