kbuild: fix a.out.h export to userspace with O= build.

We need to check for existence of the a.out.h header in the source tree,
not the object tree, if we want it to get the right answer with O=.
Signed-off-by: David Woodhouse <david.woodhouse@intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

include/asm-generic/Kbuild.asm

 header-y  += kvm.h
 
-ifeq ($(wildcard include/asm-$(SRCARCH)/a.out.h),include/asm-$(SRCARCH)/a.out.h)
+ifneq ($(wildcard $(srctree)/include/asm-$(SRCARCH)/a.out.h),)
 unifdef-y += a.out.h
 endif
 unifdef-y += auxvec.h

include/asm-powerpc/Kbuild

 include include/asm-generic/Kbuild.asm
 
-header-y += a.out.h
 header-y += auxvec.h
 header-y += ioctls.h
 header-y += mman.h

include/linux/Kbuild

@@ -166,7 +166,7 @@ unifdef-y += acct.h
 unifdef-y += adb.h
 unifdef-y += adfs_fs.h
 unifdef-y += agpgart.h
-ifeq ($(wildcard include/asm-$(SRCARCH)/a.out.h),include/asm-$(SRCARCH)/a.out.h)
+ifneq ($(wildcard $(srctree)/include/asm-$(SRCARCH)/a.out.h),)
 unifdef-y += a.out.h
 endif
 unifdef-y += apm_bios.h