kbuild: set -fconserve-stack option for gcc 4.5

The upcomming gcc 4.5 has a new -fconserve-stack option that tells the
inliner to take stack frame size in account.  Set it if the compiler
supports it.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>