[gn build] add build file for tsan runtime

Linux-only for now. Some mac bits stubbed out, but not tested.

Good enough for the tiny_race.c example at
https://clang.llvm.org/docs/ThreadSanitizer.html :

   $ out/gn/bin/clang -fsanitize=address -g -O1 tiny_race.c
   $ while true; do ./a.out || echo $? ; done

While here, also make `-fsanitize=address` work for .c files.

Differential Revision: https://reviews.llvm.org/D99795