Allow building libFuzzer in two-stage compiler-rt build

When LLVM_BUILD_EXTERNAL_COMPILER_RT option is set to true,
all of projects in compiler-rt are built with a freshly-built compiler using
a recursive CMake invocation.
(e.g. that's how compiler-rt is used in Swift)

Just now I have noticed that libFuzzer binaries were missing in such a case,
and ninja fuzzer returned "no such target", while ninja asan worked just fine.

To my surprise, the list of allowed targets was actually hardcoded in Clang!
While the current setup is clearly suboptimal, for the lack of a better fix
I'm just adding `fuzzer` to a list of `compiler-rt` targets.

Differential Revision: https://reviews.llvm.org/D38904

llvm-svn: 315771