linux-user: Add cpu loop for AArch64

Add the main linux-user cpu loop for AArch64. Since AArch64
has a different system call interface, doesn't need to worry
about FPA emulation and may in the future keep the prefetch/data
abort information in different system registers, it's simplest
just to use a completely separate loop from the 32 bit ARM
target, rather than peppering it with ifdefs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1378235544-22290-14-git-send-email-peter.maydell@linaro.org
