c++: Fix langspecs with -fsyntax-only [PR98591]

-fsyntax-only is handled specially in the driver and causes it to add
 '-o /dev/null' (or a suitable OS-specific variant thereof).  PCH is
 handled in the language driver.  I'd not sufficiently protected the
 -fmodule-only action of adding a dummy assembler from the actions of
 -fsyntax-only, so we ended up with two -o options.

	PR c++/98591
	gcc/cp/
	* lang-specs.h: Fix handling of -fmodule-only with -fsyntax-only.