[Driver] Add driver support for Fuchsia

Provide toolchain and tool support for Fuchsia operating system.
Fuchsia uses compiler-rt as the runtime library and libc++, libc++abi
and libunwind as the C++ standard library. lld is used as a default
linker.

Differential Revision: https://reviews.llvm.org/D25117

llvm-svn: 283420