[PATCH 1/3] kbuild: add clang-version.sh

+# clang-version [-p] clang-command
+#
+# Prints the compiler version of `command' in a canonical 4-digit form

small nit: `command` should be `clang-command`, but its just a comment
(maybe the maintainer can make that change when/if applying).

The conditional at the end to see if $with_patchlevel using "x" is
odd, but this is what scripts/gcc-version.sh does and this patch does
work as expected.

Tested-by: Nick Desaulniers <ndesaulniers@google.com>