x86: fix information leak on AMD CPUs

x86: fix information leak on AMD CPUs

The fix for XSA-52 was wrong, and so was the change synchronizing that
new behavior to the FXRSTOR logic: AMD's manuals explictly state that
writes to the ES bit are ignored, and it instead gets calculated from
the exception and mask bits (it gets set whenever there is an unmasked
exception, and cleared otherwise). Hence we need to follow that model
in our workaround.

This is CVE-2016-3158 / CVE-2016-3159 / XSA-172.
[xen/arch/x86/xstate.c:xrstor: CVE-2016-3158]
[xen/arch/x86/i387.c:fpu_fxrstor: CVE-2016-3159]

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>