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: NAndi Kleen <ak@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Makefile

@@ -615,6 +615,9 @@ KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
 # revert to pre-gcc-4.4 behaviour of .eh_frame
 KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
 
+# conserve stack if available
+KBUILD_CFLAGS   += $(call cc-option,-fconserve-stack)
+
 # Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
 # But warn user when we do so
 warn-assign = \