amdgcn: Delete inactive libfuncs

The HImode libfuncs weren't called and trying to enable them fails because
TARGET_PROMOTE_FUNCTION_MODE wants to widen the arguments but the signedness
isn't known.

libgcc/ChangeLog:

	* config/gcn/lib2-gcn.h (QItype, UQItype, HItype, UHItype): Delete.
	(__divhi3, __modhi3, __udivhi3, __umodhi3): Delete.
	* config/gcn/t-amdgcn: Don't build lib2-divmod-hi.c.
	* config/gcn/lib2-divmod-hi.c: Removed.