kbuild: remove GCC_VERSION

This was causing some ordering problems.  Remove the up-front evaluation
and just revaluate the compiler version each time we need it.

(The up-front evaluation was problematic because some architectures modify
the value of $(CC)).

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>