Add bfd_link_pde, and simplify some tests of link_info.type

Add bfd_link_pde, and simplify some tests of link_info.type

include/
	* bfdlink.h (bfd_link_pde): Define.
bfd/
	* elf-s390-common.c: Simplify expressions using
	bfd_linke_executable, bfd_link_pie and bfd_link_pic.
	* elf32-arm.c: Likewise.
	* elf32-bfin.c: Likewise.
	* elf32-frv.c: Likewise.
	* elf32-m68k.c: Likewise.
	* elf32-nios2.c: Likewise.
	* elf32-ppc.c: Likewise.
	* elf32-s390.c: Likewise.
	* elf32-sh.c: Likewise.
	* elf64-alpha.c: Likewise.
	* elf64-ppc.c: Likewise.
	* elf64-s390.c: Likewise.
	* elflink.c: Likewise.