arm64: emulate the swp/swpb instruction

The swp and spwb instructions were deprecated in ARMv6. ARMv8
obsoleted the instruction. Despite this, many applications rely on
these instruuctions.

This patch starts with the version present in the arm architecture.
However, it uses the ldx*()/stx*() functions to implement the handler
in C code. It also removes a lot of code that is not needed.

Change-Id: I6882fbe5f71bfa8f9e9a75d067b2111188c6f2fa
Signed-off-by: Alex Van Brunt <avanbrunt@nvidia.com>
Reviewed-on: http://git-master/r/366097


Reviewed-by: Richard Wiley <rwiley@nvidia.com>
Tested-by: Oskari Jaaskelainen <oskarij@nvidia.com>

Conflicts:
	arch/arm64/Kconfig
	arch/arm64/kernel/Makefile