x86: mitigate swapgs Spectre V1 attacks

See CVE-2019-1125. We mitigate this by adding an 'lfence'
upon interrupt/exception entry after the decision has been
made whether it's necessary to invoke 'swapgs' or not.

Only applies to x86_64, 32-bit doesn't use swapgs.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>