MIPS: Fix wrong CHECKFLAGS (sparse builds) with GCC 5.1

GCC 5.1 defines __REGISTER_PREFIX__ to $. This will break sparse
command line (and build fails with: /bin/sh: syntax error:
unexpected "(") since make tries to expand starting with the dollar
sign with a make variable. Prevent that by using double dollar sign.
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10025/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Makefile

@@ -277,7 +277,7 @@ LDFLAGS			+= -m $(ld-emul)
 ifdef CONFIG_MIPS
 CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
 	egrep -vw '__GNUC_(|MINOR_|PATCHLEVEL_)_' | \
-	sed -e "s/^\#define /-D'/" -e "s/ /'='/" -e "s/$$/'/")
+	sed -e "s/^\#define /-D'/" -e "s/ /'='/" -e "s/$$/'/" -e 's/\$$/&&/g')
 ifdef CONFIG_64BIT
 CHECKFLAGS		+= -m64
 endif