Add -lstdc++fs for ports-gcc

Index: tools/CMakeLists.txt
--- tools/CMakeLists.txt.orig
+++ tools/CMakeLists.txt
@@ -86,7 +86,7 @@ if (NOT ${SPIRV_SKIP_EXECUTABLES})
                            util/cli_consumer.cpp
                            io.cpp
                            ${COMMON_TOOLS_SRCS}
-                      LIBS ${SPIRV_TOOLS_FULL_VISIBILITY})
+                      LIBS ${SPIRV_TOOLS_FULL_VISIBILITY} ${ADDITIONAL_LIBRARIES})
     target_include_directories(spirv-objdump PRIVATE ${spirv-tools_SOURCE_DIR}
                                                      ${SPIRV_HEADER_INCLUDE_DIR})
     set(SPIRV_INSTALL_TARGETS ${SPIRV_INSTALL_TARGETS} spirv-objdump)
