[profile] Support profiling runtime on Fuchsia

This ports the profiling runtime on Fuchsia and enables the
instrumentation. Unlike on other platforms, Fuchsia doesn't use
files to dump the instrumentation data since on Fuchsia, filesystem
may not be accessible to the instrumented process. We instead use
the data sink to pass the profiling data to the system the same
sanitizer runtimes do.

Differential Revision: https://reviews.llvm.org/D47208

llvm-svn: 337881