[Fuzzer] Parametrize add_custom_libcxx

add_custom_libcxx uses the just built compiler and installs the
built libc++, e.g. for testing, neither of which is desirable in
case of Fuzzer where the libc++ should be built using the host
compiler and it's only linked into the libFuzzer and should never
be installed. This change introduces additional arguments to
add_custom_libcxx to allow parametrizing its behavior.

Differential Revision: https://reviews.llvm.org/D42330

llvm-svn: 323032