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

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>