backends: add ARM compatible mode to AARCH64

Add the initial register setup for AARCH64 running ARM code (so
called compat mode). This makes 'eu-stack -p' happy on ARM binaries
while running on a AARCH64 kernel.

Signed-off-by: Jean Pihet <jean.pihet@linaro.org>
Signed-off-by: Mark Wielaard <mjw@redhat.com>