[x86-64] (unw_step): Improve handling of functions which lack unwind info.
[x86-64] (unw_step): Improve handling of functions which lack unwind info.

If following the frame-chain leads to a frame >= 4KB away, we conclude that
the frame-pointer isn't really valid.

tony/libunwind,geekboxzone/lollipop_external_libunwind,mpercy/libunwind,tkelman/libunwind,olibc/libunwind,fillexen/libunwind,bo-on-software/libunwind,fdoray/libunwind,SyndicateRogue/libunwind,cloudius-systems/libunwind,0xlab/0xdroid-external_libunwind,Keno/libunwind,frida/libunwind,lat/libunwind,cms-externals/libunwind,dreal-deps/libunwind,rogwfu/libunwind,vtjnash/libunwind,frida/libunwind,vegard/libunwind,dreal-deps/libunwind,tronical/libunwind,bo-on-software/libunwind,dagar/libunwind,android-ia/platform_external_libunwind,igprof/libunwind,ehsan/libunwind,zeldin/platform_external_libunwind,vegard/libunwind,DroidSim/platform_external_libunwind,dropbox/libunwind,DroidSim/platform_external_libunwind,joyent/libunwind,igprof/libunwind,zliu2014/libunwind-tilegx,fdoray/libunwind,SyndicateRogue/libunwind,lat/libunwind,maltek/platform_external_libunwind,libunwind/libunwind,pathscale/libunwind,maltek/platform_external_libunwind,tkelman/libunwind,project-zerus/libunwind,project-zerus/libunwind,djwatson/libunwind,android-ia/platform_external_libunwind,cms-externals/libunwind,0xlab/0xdroid-external_libunwind,unkadoug/libunwind,igprof/libunwind,0xlab/0xdroid-external_libunwind,pathscale/libunwind,adsharma/libunwind,djwatson/libunwind,libunwind/libunwind,ehsan/libunwind,zeldin/platform_external_libunwind,jrmuizel/libunwind,tony/libunwind,android-ia/platform_external_libunwind,rantala/libunwind,dropbox/libunwind,dagar/libunwind,djwatson/libunwind,martyone/libunwind,geekboxzone/lollipop_external_libunwind,cms-externals/libunwind,Chilledheart/libunwind,ehsan/libunwind,dreal-deps/libunwind,krytarowski/libunwind,unkadoug/libunwind,DroidSim/platform_external_libunwind,geekboxzone/mmallow_external_libunwind,rogwfu/libunwind,wdv4758h/libunwind,krytarowski/libunwind,wdv4758h/libunwind,lat/libunwind,jrmuizel/libunwind,rntz/libunwind,joyent/libunwind,androidarmv6/android_external_libunwind,martyone/libunwind,yuyichao/libunwind,fillexen/libunwind,krytarowski/libunwind,zliu2014/libunwind-tilegx,tkelman/libunwind,jrmuizel/libunwind,olibc/libunwind,unkadoug/libunwind,maltek/platform_external_libunwind,martyone/libunwind,mpercy/libunwind,wdv4758h/libunwind,rogwfu/libunwind,evaautomation/libunwind,adsharma/libunwind,rantala/libunwind,adsharma/libunwind,evaautomation/libunwind,atanasyan/libunwind-android,atanasyan/libunwind-android,mpercy/libunwind,rntz/libunwind,CyanogenMod/android_external_libunwind,tronical/libunwind,atanasyan/libunwind,zeldin/platform_external_libunwind,CyanogenMod/android_external_libunwind,dropbox/libunwind,joyent/libunwind,Chilledheart/libunwind,rantala/libunwind,SyndicateRogue/libunwind,androidarmv6/android_external_libunwind,geekboxzone/mmallow_external_libunwind,rntz/libunwind,fillexen/libunwind,geekboxzone/lollipop_external_libunwind,CyanogenMod/android_external_libunwind,vtjnash/libunwind,zliu2014/libunwind-tilegx,fdoray/libunwind,dagar/libunwind,frida/libunwind,cloudius-systems/libunwind,pathscale/libunwind,geekboxzone/mmallow_external_libunwind,androidarmv6/android_external_libunwind,evaautomation/libunwind,bo-on-software/libunwind,atanasyan/libunwind-android,project-zerus/libunwind,olibc/libunwind,cloudius-systems/libunwind,tronical/libunwind,vegard/libunwind,atanasyan/libunwind,vtjnash/libunwind,libunwind/libunwind,tony/libunwind,Keno/libunwind,Keno/libunwind,Chilledheart/libunwind,atanasyan/libunwind,yuyichao/libunwind,yuyichao/libunwind