Wean gdb and sim off private libbfd.h header

Wean gdb and sim off private libbfd.h header

The major reason this header was needed, bfd_default_set_arch_mach,
has now moved to bfd.h.

gdb/
	* amd64-darwin-tdep.c: Don't include libbfd.h.
	* i386-darwin-tdep.c: Likewise.
	* rs6000-nat.c: Likewise.
	* rs6000-tdep.c: Likewise.
sim/aarch64/
	* memory.c: Don't include libbfd.h.
sim/rl78/
	* load.c: Don't include libbfd.h.
	(rl78_load): Don't use private iovec seek or read.
sim/rx/
	* load.c: Don't include libbfd.h.
	(rx_load): Don't use private iovec seek or read.