kbuild: remove ar-option and KBUILD_ARFLAGS

Commit 40df759e2b9e ("kbuild: Fix build with binutils <= 2.19")
introduced ar-option and KBUILD_ARFLAGS to deal with old binutils.

According to Documentation/process/changes.rst, the current minimal
supported version of binutils is 2.21 so you can assume the 'D' option
is always supported. Not only GNU ar but also llvm-ar supports it.

With the 'D' option hard-coded, there is no more user of ar-option
or KBUILD_ARFLAGS.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 4a0cdd6..179d55a 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -232,7 +232,7 @@
 # ---------------------------------------------------------------------------
 
 quiet_cmd_ar = AR      $@
-      cmd_ar = rm -f $@; $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(real-prereqs)
+      cmd_ar = rm -f $@; $(AR) cDPrsT $@ $(real-prereqs)
 
 # Objcopy
 # ---------------------------------------------------------------------------