Implement __gnu_[u]ldivmod_helper methods in libc

These symbols were previously provided by Android's out-dated libgcc,
but they're removed/deprecated in upstream libgcc, and also won't be
available in libclang_rt.builtins. We need to provide these methods in
libc itself for compatiblity.

Test: build with these symbols stripped in libgcc
Bug: 29275768
Change-Id: Ie1ccdb711872bf3fc317cb908fed67c9a8955e42
Merged-In: I04a05258c6c06b5a22ead41e148b02792ffbc941
(cherry picked from commit b410d0e69ef6f545c7652ee0e4cf7b4dd4d2c38a)