* rename is_compiler_rt_or_libc to skip_linker_dependencies
   and set it to `true` for all sub-Compilations. I believe
   this resolves the deadlock we were experiencing on Drone
   CI and on some users' computers. I will remove the CI workaround in
   a follow-up commit.
 * enabling TSAN automatically causes the Compilation to link against
   libc++ even if not requested, because TSAN depends on libc++.
 * add -fno-rtti flags where appropriate when building TSAN objects.
   Thanks Firefox317 for pointing this out.
 * TSAN support: resolve all the undefined symbols. We are still seeing
   a dependency on __gcc_personality_v0 but will resolve this one in a
   follow-up commit.
 * static libs do not try to build libc++ or libc++abi.