[x86-64] Fix cross-compilation.  setcontext.S can only be built natively.
[x86-64] Fix cross-compilation.  setcontext.S can only be built natively.

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