xen: Have Kconfig check $(CC)'s version

xen: Have Kconfig check $(CC)'s version

This import several files from Linux v5.3
 - scripts/Kconfig.include
 - scripts/clang-version.sh
 - scripts/gcc-version.sh
 and several config values from from Linux's init/Kconfig file.
But gcc-version.sh have been modified to return "0" when $CC isn't
GCC, like clang-version.sh do.

Files are copied into scripts/ directory because that's were the files
are found in Linux tree, and also because we are going to import more
of Kbuild from Linux which is located in scripts/.

CONFIG_GCC_VERSION and CONFIG_CC_IS_CLANG are going to be use in
follow-up patches.

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