Add check for XSP in dr_restore_reg and dr_save_reg on AArch64

Created by: yury-khrustalev

This check is helpful for AArch64 target which does not support loading into and storing from XSP. This change allows to fail early with a meaningful error message: for a debug build it will look like this:

<Application /usr/bin/ls (68183) DynamoRIO usage error : dr_save_reg: store from XSP is not supported>
<Usage error: dr_save_reg: store from XSP is not supported (...dynamorio/core/lib/instrument.c, line 5807)