Ada: Fix s-oscons.ads generation

Ada: Fix s-oscons.ads generation

The $(GNATLIBCFLAGS) are already included in $(GNATLIBCFLAGS_FOR_C).

We must call the C compiler with the right machine flags.  So, add
$(GNATLIBCFLAGS_FOR_C) to $(OSCONS_EXTRACT).  For example, on a bi-arch
compiler supporting 32-bit and 64-bit instruction sets we pick otherwise
only one variant due to a missing -m32 or -m64 flag.

gcc/ada
	* gcc-interface/Makefile.in (OSCONS_CPP): Remove redundant
	$(GNATLIBCFLAGS).
	(OSCONS_EXTRACT): Add $(GNATLIBCFLAGS_FOR_C).

From-SVN: r258334