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 <[e-mail]>
Cc: Andrew Bresticker <[e-mail]>
Cc: Alex Smith <[e-mail]>
Cc: Wu Zhangjin <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/12232/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/boot/compressed/Makefile

···
 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))