aarch64: Avoid registering duplicate C++ overloads [PR112989]

aarch64: Avoid registering duplicate C++ overloads [PR112989]

In the original fix for this PR, I'd made sure that
including <arm_sme.h> didn't reach the final return in
simulate_builtin_function_decl (which would indicate duplicate
function definitions).  But it seems I forgot to do the same
thing for C++, which defines all of its overloads directly.

This patch fixes a case where we still recorded duplicate
functions for C++.  Thanks to Iain for reporting the resulting
GC ICE and for help with reproducing it.

gcc/
	PR target/112989
	* config/aarch64/aarch64-sve-builtins-shapes.cc (build_one): Skip
	MODE_single variants of functions that don't take tuple arguments.