Add aarch64 getcontext functionality.
Add aarch64 getcontext functionality.

Change-Id: I32912a85c0fd02bb5e45a9eb7deb2410ded352a9

geekboxzone/lollipop_external_libunwind,olibc/libunwind,zeldin/platform_external_libunwind,frida/libunwind,zeldin/platform_external_libunwind,atanasyan/libunwind-android,CyanogenMod/android_external_libunwind,geekboxzone/mmallow_external_libunwind,atanasyan/libunwind-android,maltek/platform_external_libunwind,olibc/libunwind,zeldin/platform_external_libunwind,geekboxzone/lollipop_external_libunwind,android-ia/platform_external_libunwind,androidarmv6/android_external_libunwind,frida/libunwind,android-ia/platform_external_libunwind,DroidSim/platform_external_libunwind,olibc/libunwind,CyanogenMod/android_external_libunwind,maltek/platform_external_libunwind,SyndicateRogue/libunwind,SyndicateRogue/libunwind,geekboxzone/mmallow_external_libunwind,androidarmv6/android_external_libunwind,androidarmv6/android_external_libunwind,SyndicateRogue/libunwind,DroidSim/platform_external_libunwind,geekboxzone/lollipop_external_libunwind,frida/libunwind,DroidSim/platform_external_libunwind,geekboxzone/mmallow_external_libunwind,atanasyan/libunwind-android,maltek/platform_external_libunwind,android-ia/platform_external_libunwind,CyanogenMod/android_external_libunwind