Use -Wno-pointer-sign for gcc 4.0

Compiling an allyesconfig kernel straight with a gcc 4.0 snapshot gives nearly 10k new warnings like:

warning: pointer targets in passing argument 5 of `cpuid' differ in signedness

Since the sheer number of these warnings was too much even for the most determined kernel janitors (I actually asked ;-) and I don't think it's a very serious issue to have these mismatches I submitted an new option to gcc to disable it. It was incorporated in gcc mainline now.

This patch makes the kernel compilation use it. There are still quite a lot of new warnings with 4.0 (mostly about uninitialized variables), but the compile log looks much nicer nnow.

@@ -533,6 +533,9 @@
 # warn about C99 declaration after statement
 CFLAGS += $(call cc-option,-Wdeclaration-after-statement,)
 
+# disable pointer signedness warnings in gcc 4.0
+CFLAGS += $(call cc-option,-Wno-pointer-sign,)
+
 # Default kernel image to build when no specific target is given.
 # KBUILD_IMAGE may be overruled on the commandline or
 # set in the environment