efi/x86: Replace #ifdefs with IS_ENABLED() checks

When possible, IS_ENABLED() conditionals are preferred over #ifdefs,
given that the latter hide the code from the compiler entirely, which
reduces build test coverage when the option is not enabled.

So replace an instance in the x86 efi startup code.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>