d: Fix order of precedence for -defaultlib and -debuglib

The order of precedence used by the upstream reference compiler for
determining what library to link against is:
- No library if -nophoboslib or -fno-druntime was seen.
- The library passed to -debuglib if -g was also seen.
- The library passed to -defaultlib
- The in-tree libgphobos library.

This aligns the D language driver to follow the same rules.

gcc/d/ChangeLog:

	* d-spec.cc (need_phobos): Remove.
	(lang_specific_driver): Replace need_phobos with phobos_library.
	Reorder -debuglib and -defaultlib to have precedence over libphobos.
	(lang_specific_pre_link): Remove test for need_phobos.