target-i386: Remove confusing X86_64_DEF macro

The X86_64_DEF macro is a confusing way of making some terms
in a conditional only appear if TARGET_X86_64 is defined. We
only use it in two places, and in both cases this is for making
the same test, so abstract that check out into a function
where we can use a more conventional #ifdef.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
