arm64: fix SWP instruction emulation

arm64: fix SWP instruction emulation

initial variable values may get overwritten
if they're listed as an output in ASM, even if
they're not explicitly written to.

Change-Id: I2a239e1819850a2a7005a46e83d82deac4ca303b
Signed-off-by: Rich Wiley <rwiley@nvidia.com>
Reviewed-on: http://git-master/r/379646
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Li Li (SW-TEGRA) <lli5@nvidia.com>
Tested-by: Li Li (SW-TEGRA) <lli5@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Alexander Van Brunt <avanbrunt@nvidia.com>