kbuild: fix modpost compile with older gcc

The kernel now requires that CC be 3.1.0 or higher.  But we shouldn't place
that requirement upon HOSTCC unless we really need to.  Fixes my ia64 problem.

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