configure: Add handling code for AArch64 targets

Add the necessary code to configure to handle AArch64 as a target
CPU (we already have some code for supporting it as host). Note
that this doesn't enable the AArch64 targets yet.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: John Rigby <john.rigby@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1378235544-22290-23-git-send-email-peter.maydell@linaro.org
Message-id: 1368505980-17151-12-git-send-email-john.rigby@linaro.org
[PMM:
 * don't need to set TARGET_ABI_DIR to aarch64 as that is the default
 * don't build nwfpe -- this is 32 bit legacy only
 * rewrite commit message
 * add aarch64 to the list of "fdt required" targets
]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>