x86/shim: don't use 32-bit compare on boolean variable

x86/shim: don't use 32-bit compare on boolean variable

Current upstream gas silently assumes 32-bit operand size for most
operations where the size can't be inferred from an involved register
(my own one doesn't anymore, which is how I've noticed this). It is pure
luck that the 3 bytes following pvh_boot are currently padding ones.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>