Darwin, Simplify headers 4/5 : Remove redundant headers.

The darwinN.h headers (with the sole exception of darwin7.h,
which contains a target macro definition) now only contain
values that set fall-backs for cross-compilations, these can
be provided from the config.gcc script which means we no longer
need the darwinN.h - so delete them.

gcc/ChangeLog:

	* config.gcc: Compute default version information
	from the configured target.  Likewise defaults for
	ld64.
	* config/darwin10.h: Removed.
	* config/darwin12.h: Removed.
	* config/darwin9.h: Removed.
	* config/rs6000/darwin8.h: Removed.