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.