[CMake] LLVM_PLUGIN_EXT: Use CMAKE_SHARED_LIBRARY_SUFFIX rather than...

[CMake] LLVM_PLUGIN_EXT: Use CMAKE_SHARED_LIBRARY_SUFFIX rather than CMAKE_SHARED_MODULE_SUFFIX in llvm tree.

FIXME: llvm/test may be aware of LLVM_PLUGIN_EXT, like as clang/test does.
FIXME: CMAKE_*_SUFFIX may be set in HandleLLVMOptions if those variables could be writable, rather than to set one as target properties.
llvm-svn: 201316