Add support for AArch64 CloudABI binaries.

Add support for AArch64 CloudABI binaries.

ld	* Makefile.am (ALL_64_EMULATION_SOURCES): Add support for
	CloudABI on aarch64. For this target we have to make sure we use
	ELFOSABI_CLOUDABI instead of ELFOSABI_NONE.
	* configure.tgt (targ_emul): Likewise.
	* emulparams/aarch64cloudabi.sh: New file.
	* emulparams/aarch64cloudabib.sh: New file.
	* Makefile.in: Regenerate.

bfd	* config.bfd (targ_defvec): Add support for CloudABI on aarch64.
	For this target we have to make sure we use ELFOSABI_CLOUDABI
	instead of ELFOSABI_NONE.
	* configure.ac (tb): Likewise.
	* elfnn-aarch64.c: Likewise.
	* targets.c (_bfd_target_vector): Likewise.
	* configure: Regenerate.

gas	* config/tc-aarch64.c (elf64_aarch64_target_format): Select the
	cloudabi format if the TARGET_OS is cloudabi.