validate_guest_space incorrectly enabled on AArch64

When running linux-user targetting AArch64, validate_guest_space() in elfload.c reserves space in the guest address space for the ARM commpage. Since there is no commpage on AArch64, this function should be disable on that target.

Thanks for the bug report -- I've just submitted this patch which should fix this:
  http://patchwork.ozlabs.org/patch/328565/


Fix will be in 2.0.