i#4508 Dynamorio not loading stolen register value into mcontext on AArch64.
On AArch64 the value of the stolen register isn't loaded into the mcontext object for later instrumentation.
This is due to an ifdef only allowing this functionality on ARM (non AArch64), in core/lib/instrument.c, dr_get_mcontext_priv().
This causes errors when addresses need to be calculated based on the value of contents the stolen register.
Fix is a simple update of an #ifdef from #ifdef ARM to ifdef AARCHXX