Add support for the aarch64-gnu target (GNU/Hurd on AArch64)

Add support for the aarch64-gnu target (GNU/Hurd on AArch64)

Also recognized are aarch64-*-gnu tagrets, e.g. aarch64-pc-gnu or
aarch64-unknown-gnu.

The ld/emulparams/aarch64gnu.sh file is (for now) identical to aarch64fbsd.sh,
or to aarch64linux.sh with Linux-specific logic removed; and mainly different
from the generic aarch64elf.sh in that it does not set EMBEDDED=yes.

Coupled with a corresponding GCC patch, this produces a toolchain that can
sucessfully build working binaries targeting aarch64-gnu.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>