Attached is a patch that makes ocaml able to compile proper PIC code on amd64.
This allows shared libraries to be created from ocaml code.

This patch needs work - in addition to being rough around the edges, there is
a fairly major issue where it will generate patters as such:

some_label:
     movq camlPcre@GOTPCREL(%rip), %r13
     movq 248(%r13), %r10
     call caml_apply8@PLT
...
caml_apply8:
    call (%r10) # should call camlPcre+248

This would be cool but %r10 is clobbered by the call in some circumstances as
it's defined as a scratch register.  Ocaml's compiler seems to have logic to
'spill' registers onto the stack but I haven't studied it hard enough to put
it to use; maybe someone else is more familiar and will be able to mark
registers as live when they are used in this way?  :)

- Erik