xen: Import cc-ifversion from Kbuild

xen: Import cc-ifversion from Kbuild

This is in preparation of importing Kbuild to build Xen. We won't be
able to include Config.mk so we will need a replacement for the macro
`cc-ifversion'.

This patch imports parts of "scripts/Kbuild.include" from Linux v5.4,
the macro cc-ifversion. It makes use of CONFIG_GCC_VERSION that
Kconfig now provides.

Since they are no other use of Xen's `cc-ifversion' macro, we can
remove it.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>