ARM64 also supports float point LDP(load pair) & STP (store pair).
The CL adds implementation and corresponding test cases for
FLDPD/FLDPS/FSTPD/FSTPS.

Change-Id: I45f112012a4e097bfaf023d029b36e6cbc7a5859
Reviewed-on: https://go-review.googlesource.com/125438
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>