amdgcn: Remove TARGET_GCN3

The only GCN3 ISA device was remove (Fiji, gfx803) so all the GCN3-specific
code and features can be removed from the back-end.

gcc/ChangeLog:

	* config/gcn/gcn-opts.h (enum gcn_isa): Delete ISA_GCN3.
	(TARGET_GCN3): Delete.
	(TARGET_GCN3_PLUS): Delete.
	(TARGET_M0_LDS_LIMIT): Delete.
	* config/gcn/gcn-valu.md
	(gather<mode>_insn_1offset<exec>): Remove TARGET_GCN3 from conditions.
	(*<reduc_op>_dpp_shr_<mode>): Likewise.
	* config/gcn/gcn.cc (enum gcn_isa): Change default to ISA_GCN5.
	(gcn_expand_prologue): Remove TARGET_M0_LDS_LIMIT feature.
	(gcn_expand_reduc_scalar): Remove TARGET_GCN3 conditions.
	* config/gcn/gcn.h (TARGET_CPU_CPP_BUILTINS): Remove TARGET_GCN3.