The ability to partially link libcxx into fuzzer does not work for cross
compilations. CMAKE_EXE_LINKER_FLAGS, etc are not passed to the custom CMake
invocation and cross-compiling libcxx without a libcxxabi, as is done here, has
further issues as well.
Add an option to disable linking of libcxx into fuzzer. The default for this
option is to link libcxx.