[fuzzer] Update and enable libFuzzer on Fuchsia

This change updates the Fuchsia-specific code to use the C++ friendly
duration expressions and flips on the building of
libclang_rt.fuzzer-x86_64.a and similar for Fuchsia. Given that
compiler-rt doesn't build on Fuchsia, test have been run by explicitly
building the library and linking it against
lib/fuzzer/tests/FuzzerUnittest.cpp.

Differential Revision: https://reviews.llvm.org/D42670

llvm-svn: 323828