Move GNU/Hurd startfile spec from config/i386/gnu.h to config/gnu.h

Move GNU/Hurd startfile spec from config/i386/gnu.h to config/gnu.h

Since it's not i386-specific; this makes it possible to reuse it for other
architectures.

Also, add a warning for the case gnu.h is specified before gnu-user.h, which
would cause gnu-user's version of the spec to override gnu's, and not the other
way around as it's intended. The i?86-gnu target currently specifies them in
the right order, but it's easy to accidentally put them in a wrong order.

gcc/Changelog:

	* config/i386/gnu.h: Move GNU/Hurd STARTFILE_SPEC from here...
	* config/gnu.h: ...to here.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>