x86-64: Simplify minimum ISA check ifdef conditional with if

Replace minimum ISA check ifdef conditional with if.  Since
MINIMUM_X86_ISA_LEVEL and AVX_X86_ISA_LEVEL are compile time constants,
compiler will perform constant folding optimization, getting same
results.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
(cherry picked from commit b6e38981)