Allow chisel_emulator.h to build undere -Wall

I like to build my code with -Wall.  While it appears that the
generated Chisel C++ compiles OK (though I haven't tested this
extensively yet) the hand-coded header file doesn't.

This takes advantage of GCC's "#pragma GCC diagnostic" to hide the
warnings that GCC generates for me.  This depends on having a new-ish
GCC (because of the push/pop pragmas), but it does allow me to have
GCC generate warnings for my code while still hiding the warnings that
come from Chisel's header.