MIPS: zboot: Fix the build with XZ compression on older GCC versions

Some older GCC version (at least 4.6) emits calls to __bswapsi2() when
building the XZ decompressor. The link of the compressed image then
fails with the following error:

arch/mips/boot/compressed/decompress.o: In function '__fswab32':
include/uapi/linux/swab.h:60: undefined reference to '__bswapsi2'

Add bswapsi.o to the link to fix the build with these versions.
Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Alex Smith <alex.smith@imgtec.com>
Cc: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12232/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/boot/compressed/Makefile

@@ -39,10 +39,10 @@ vmlinuzobjs-$(CONFIG_SYS_SUPPORTS_ZBOOT_UART_PROM) += $(obj)/uart-prom.o
 vmlinuzobjs-$(CONFIG_MIPS_ALCHEMY)		   += $(obj)/uart-alchemy.o
 endif
 
-vmlinuzobjs-$(CONFIG_KERNEL_XZ) += $(obj)/ashldi3.o
+vmlinuzobjs-$(CONFIG_KERNEL_XZ) += $(obj)/ashldi3.o $(obj)/bswapsi.o
 
-$(obj)/ashldi3.o: KBUILD_CFLAGS += -I$(srctree)/arch/mips/lib
-$(obj)/ashldi3.c: $(srctree)/arch/mips/lib/ashldi3.c
+$(obj)/ashldi3.o $(obj)/bswapsi.o: KBUILD_CFLAGS += -I$(srctree)/arch/mips/lib
+$(obj)/ashldi3.c $(obj)/bswapsi.c: $(obj)/%.c: $(srctree)/arch/mips/lib/%.c
 	$(call cmd,shipped)
 
 targets := $(notdir $(vmlinuzobjs-y))