arm64: Align code and data to 8 bytes.

Insufficient alignment seems to be the cause of relocation errors when
linking large native code OCaml programs:

 (.text+0xc): relocation truncated to fit: R_AARCH64_LDST64_ABS_LO12_NC against symbol `camlOdoc_type' defined in .data section in odoc_type.o
../stdlib/stdlib.a(listLabels.o): In function `camlListLabels__entry':
(.text+0x10): relocation truncated to fit: R_AARCH64_LDST64_ABS_LO12_NC against symbol `camlListLabels' defined in .data section in ../stdlib/stdlib.a(listLabels.o)

PR#6283 http://caml.inria.fr/mantis/view.php?id=6283