DynamicLoader: Remove math functionality in favor of -lgcc

This links the dynamic linker against libgcc.a instead of having
our own copy of the math functions.

For now we need to specify -fbuilding-libgcc as a hack to work
around a bug with the -nodefaultlibs flag. Once everyone is on
the latest toolchain version this can be removed.

Userland/DynamicLoader/main.cpp

@@ -116,4 +116,10 @@ void _start(int argc, char** argv, char** envp)
     ELF::DynamicLinker::linker_main(move(main_program_name), main_program_fd, is_secure, argc, argv, envp);
     VERIFY_NOT_REACHED();
 }
+
+void _fini();
+
+void _fini()
+{
+}
 }