GCN: Don't hard-code number of SGPR/VGPR/AVGPR registers

Also add 'STATIC_ASSERT's for number of SGPR/VGPR/AVGPR registers (in
'#ifndef USED_FOR_TARGET', as otherwise 'STATIC_ASSERT' isn't available).

	gcc/
	* config/gcn/gcn.cc (gcn_hsa_declare_function_name): Don't
	hard-code number of SGPR/VGPR/AVGPR registers.
	* config/gcn/gcn.h: Add a 'STATIC_ASSERT's for number of
	SGPR/VGPR/AVGPR registers.