* 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.
