Reduce the number of ifdefs in machine independent code
Reduce the number of ifdefs in machine independent code

Hopefully we don't have too many GNUC users who don't have
fetch and add.

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