Please note the question was about compiled libstdc++ having SSE2 instructions in the object code, so the question was not about gcc command line arguments. No matter what I'm supplying to gcc when compiling my code, it will link with libstdc++ that already has built-in SSE2 instructions.
The real answer is not to use ANY --with-fpmath switches when compiling GCC. I got confused by the configure script switch statement thinking that it only supports sse or avx, while, in fact, the default value (not mentioned in this switch is "387"). So make sure you don't use --with-fpmath when running configure. I recompiled GCC without it and it now works fine.