[clang-repl] Fix CMake errors when cross compiling

These stem from 4821c90c.

When cross compiling, CMAKE_SYSTEM_PROCESSOR is empty, if the
target processor hasn't been set when setting up the cross
compilation. Ideally, when setting up cross compilation with
CMake, the user is supposed to set CMAKE_SYSTEM_PROCESSOR, but
so far, compilation has worked just fine even without it.

Quote the string where CMAKE_SYSTEM_PROCESSOR is expanded, to
avoid argument errors when it expands to an empty string.