i#1566 software divide on ARM: use libgcc divide emulation code

Adds gcc runtime code under third_party/libgcc to supply software divide
emulation for ARM, where we can't rely on hardware integer divide support.
The GPL "GCC Runtime Library Exception" allows us to link these files into
our BSD library.

Imports 4 assembly files and 1 extracted C routine from libgcc.  The
assembly files are built in separate passes due to label conflicts.

Fixes #1566

Review-URL: https://codereview.appspot.com/186420043