protect some limit constants with feature test macros on x86_64
protect some limit constants with feature test macros on x86_64

rofl0r/musl-old,chardi/musl,chneukirchen/musl-chris2,chardi/musl,chardi/musl,rofl0r/musl-old,davidlazar/musl,davidlazar/musl,chneukirchen/musl-chris2