This is a follow-up to CL 5978051.
    Use kernel cas64 helper if we can, fallback to LDREXD/STREXD if
    we are on ARMv6 or higher, and to lock-emulated cas64 if on ARMv5.
    A future CL will fix {Add,Load,Store}{Int,Uint}64 and issue 3331.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6034048