aarch64: Simplify AARCH64_STACK_ALIGN
aarch64: Simplify AARCH64_STACK_ALIGN

The iOS abi doesn't require padding between arguments, but
that's not what AARCH64_STACK_ALIGN meant.  The hardware will
in fact trap if the SP register is not 16 byte aligned.

Pan7/libffi,Distrotech/libffi,rth7680/libffi,nmav/libffi,biosbits/libffi,rth7680/libffi,s1341/libffi,joshtriplett/libffi,rth7680/libffi,bgarrels/libffi,Pan7/libffi,Pan7/libffi,plicease/libffi,nmav/libffi,plicease/libffi,Pan7/libffi,s1341/libffi,bgarrels/libffi,s1341/libffi,joshtriplett/libffi,bgarrels/libffi,biosbits/libffi,Distrotech/libffi,s1341/libffi,biosbits/libffi,biosbits/libffi,rth7680/libffi,Distrotech/libffi,joshtriplett/libffi,Distrotech/libffi,plicease/libffi,joshtriplett/libffi,bgarrels/libffi,nmav/libffi,plicease/libffi,nmav/libffi