It's not entirely clear, but I think this is a C++ only feature that is only available when using the GCC C++ front end (g++). The linked document includes the ambiguous "Support is only available in C++ for i386 targets", but other sources (such as the GCC 4.9.2 documentation) have clarified this to "With the GNU C++ front end, for target i386, you may specify multiple versions of a function".

https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Function-Multiv...