Fix clone, select, and exits

Another round of x86-64 fixes:

* `clone(2)` has a different parameter ordering on x86-64 vs. x86.  We need to handle that.
* `select(2)` has a different calling convention on x86-64 vs. x86.  (x86 also has the `newselect` syscall, which places the arguments in registers, following the x86-64 version.)  I thought about genericizing `enum MmapCallingSemantics` to handle this case, but decided against it, as `MmapCallingSemantics` might also describe other things (whether `offset` is in pages or bytes, for instance).
* `destroy_buffers` needs to handle x86-64 syscalls.  It's rather pleasant that function exists; otherwise, one would have to get the whole raft of `librrpreload` floating before you could do anything useful...