kbuild: thin archives make default for all archs

Make thin archives build the default, but keep the config option
to allow exemptions if any breakage can't be quickly solved.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/process/changes.rst

@@ -31,7 +31,7 @@ you probably needn't concern yourself with isdn4k-utils.
 ====================== ===============  ========================================
 GNU C                  3.2              gcc --version
 GNU make               3.81             make --version
-binutils               2.12             ld -v
+binutils               2.20             ld -v
 util-linux             2.10o            fdformat --version
 module-init-tools      0.9.10           depmod -V
 e2fsprogs              1.41.4           e2fsck -V
@@ -75,10 +75,9 @@ You will need GNU make 3.81 or later to build the kernel.
 Binutils
 --------
 
-Linux on IA-32 has recently switched from using ``as86`` to using ``gas`` for
-assembling the 16-bit boot code, removing the need for ``as86`` to compile
-your kernel.  This change does, however, mean that you need a recent
-release of binutils.
+The build system has, as of 4.13, switched to using thin archives (`ar T`)
+rather than incremental linking (`ld -r`) for built-in.o intermediate steps.
+This requires binutils 2.20 or newer.
 
 Perl
 ----

arch/Kconfig

@@ -511,7 +511,7 @@ config CC_STACKPROTECTOR_STRONG
 endchoice
 
 config THIN_ARCHIVES
-	bool
+	def_bool y
 	help
 	  Select this if the architecture wants to use thin archives
 	  instead of ld -r to create the built-in.o files.

arch/powerpc/Kconfig

@@ -472,14 +472,6 @@ config MPROFILE_KERNEL
 	depends on PPC64 && CPU_LITTLE_ENDIAN
 	def_bool !DISABLE_MPROFILE_KERNEL
 
-config USE_THIN_ARCHIVES
-	bool "Build the kernel using thin archives"
-	default n
-	select THIN_ARCHIVES
-	help
-	  Build the kernel using thin archives.
-	  If you're unsure say N.
-
 config IOMMU_HELPER
 	def_bool PPC64