[clang] Enable -fsanitize=thread on Fuchsia.

This CL modifies clang enabling using -fsanitize=thread on fuchsia. The
change doesn't build the runtime for fuchsia, it just enables the
instrumentation to be used.

pair-programmed-with: mdempsky@google.com
Change-Id: I816c4d240d1f15e9eae2803fb8ba3a7bf667ed51

Reviewed By: mcgrathr, phosek

Differential Revision: https://reviews.llvm.org/D86822