linux-user: Implement new ARM 64 bit cmpxchg kernel helper

linux-user: Implement new ARM 64 bit cmpxchg kernel helper

Linux 3.1 will have a new kernel-page helper for ARM implementing
64 bit cmpxchg. Implement this helper in QEMU linux-user mode:
 * Provide kernel helper emulation for 64bit cmpxchg
 * Allow guest to object to guest offset to ensure it can map a page
 * Populate page with kernel helper version

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dr. David Alan Gilbert <david.gilbert@linaro.org>