Darwin : Adjust defaults for current bootstrap constraints.

The toolchain now requires a C++11 compiler to bootstrap and
none of the older Darwin toolchains which were based on stabs
debugging are suitable.  We can simplify the debug setup now.

gcc/ChangeLog:

	* config/darwin.h (DSYMUTIL_SPEC): Default to DWARF
	(ASM_DEBUG_SPEC):Only define if the assembler supports
	stabs.
	(PREFERRED_DEBUGGING_TYPE): Default to DWARF.
	(DARWIN_PREFER_DWARF): Define.
	* config/darwin9.h (PREFERRED_DEBUGGING_TYPE): Remove.
	(DARWIN_PREFER_DWARF): Likewise
	(DSYMUTIL_SPEC): Likewise.
	(COLLECT_RUN_DSYMUTIL): Likewise.
	(ASM_DEBUG_SPEC): Likewise.
	(ASM_DEBUG_OPTION_SPEC): Likewise.