powerpc: Add option to use thin archives

Add an option to use thin archives to build the kernel.
Thin archives are explained in commit a5967db9af51 ("kbuild: allow
architectures to use thin archives instead of ld -r").

This is a gradual way to introduce the option to testers.

Some change to the way we invoke ar is required so it can be used
by scripts/link-vmlinux.sh.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make it an explicit option not dependant on COMPILE_TEST]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Kconfig

@@ -396,6 +396,14 @@ 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
 

arch/powerpc/Makefile

@@ -23,7 +23,7 @@ CROSS32AR		:= $(CROSS32_COMPILE)ar
 ifeq ($(HAS_BIARCH),y)
 ifeq ($(CROSS32_COMPILE),)
 CROSS32CC	:= $(CC) -m32
-CROSS32AR	:= GNUTARGET=elf32-powerpc $(AR)
+KBUILD_ARFLAGS	+= --target=elf32-powerpc
 endif
 endif
 
@@ -85,7 +85,7 @@ ifeq ($(HAS_BIARCH),y)
 override AS	+= -a$(BITS)
 override LD	+= -m elf$(BITS)$(LDEMULATION)
 override CC	+= -m$(BITS)
-override AR	:= GNUTARGET=elf$(BITS)-$(GNUTARGET) $(AR)
+KBUILD_ARFLAGS	+= --target=elf$(BITS)-$(GNUTARGET)
 endif
 
 LDFLAGS_vmlinux-y := -Bstatic