Use MemoryBarrier on Win64
Use MemoryBarrier on Win64

We leave the 32 bit version, tho. The `MemoryBarrier` expands to a
`__faststorefence` on x86_64, which compiles to

    lock or DWORD PTR [rsp], ebp

The 32bit code we use is quite similar but avoids the ebp access:

    lock add [esp], 0

whereas the x86 `MemoryBarrier` is a more costly `xchg [...], eax`.

OpenSmalltalk/vm,peteruhnak/pharo-vm,timfel/squeakvm,bencoman/pharo-vm,OpenSmalltalk/vm,timfel/squeakvm,bencoman/pharo-vm,peteruhnak/pharo-vm,peteruhnak/pharo-vm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,bencoman/pharo-vm,peteruhnak/pharo-vm,timfel/squeakvm,OpenSmalltalk/vm,peteruhnak/pharo-vm,OpenSmalltalk/vm,bencoman/pharo-vm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,bencoman/pharo-vm,timfel/squeakvm,bencoman/pharo-vm,peteruhnak/pharo-vm,bencoman/pharo-vm,OpenSmalltalk/vm,bencoman/pharo-vm,OpenSmalltalk/vm,bencoman/pharo-vm,peteruhnak/pharo-vm,peteruhnak/pharo-vm