[scudo] Fuchsia minimal shared runtime

Summary:
Fuchsia requires its Scudo shared runtime to not be C++ dependant. Since they
don't use UBSan in conjunction with Scudo, we can just remove the runtime,
and add the extra `nostdinc++` and `nostdlib++` flags. No need for Coverage
either. This allows to keep things going while working on additional splits
of sanitizer_commong and a more minimal runtime.

Reviewers: phosek, flowerhack, alekseyshl

Reviewed By: phosek, alekseyshl

Subscribers: mgorny, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D44791

llvm-svn: 328625