ARM64 port continued:
- arm64.S: fix saving of float regs around GC call
- asmcomp/arm64: use GOT addressing for external symbols if Clflags.dlcode
  (but not for internal, non-exported symbols, as it triggers bad linking)
- asmcomp/arm64: support "mov dN, #fpimm" insn
- byterun/interp.c: add register decls for ARM64
- otherlibs/num, configure: add small asm support for ARM64


git-svn-id: http://caml.inria.fr/svn/ocaml/branches/arm64@13905 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02