kbuild: document cc-fullversion

	The third parameter may be a text as in this example, but it may also

	be an expanded variable or a macro.

    cc-fullversion

	cc-fullversion is useful when the exact version of gcc is needed.

	One typical use-case is when a specific GCC version is broken.

	cc-fullversion points out a more specific version than cc-version does.

	Example:

		#arch/powerpc/Makefile

		$(Q)if test "$(call cc-fullversion)" = "040200" ; then \

			echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \

			false ; \

		fi

	In this example for a specific GCC version the build will error out explaining

	to the user why it stops.

=== 4 Host Program support