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

[Nathan Sidwell]

-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.	

-- 
Nathan Sidwell
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