> I was able to successfully build gcc-trunk using the provided patch.
> moreover, I was able to successfully build all of the packages used in
> the toolchain!
> (gmp, mpfr, mpc, isl, libgnurx, bzip2, termcap, libffi, expat, ncurses,
> readline, gdbm, tcl, tk, openssl, xz-utils, sqlite, python3, binutils,
> gdb, make)

Great!  Did you check that C++ threads are enabled in your build?  If they 
are, you must be able to run the attached C++ test; if they are not (because 
the MinGW64 build is configured for older versions of Windows), you need to 
configure the compiler with the option --enable-libstdcxx-threads.

-- 
Eric Botcazou