MIPS: Fix -mabi=64 build of vdso.lds

MIPS: Fix -mabi=64 build of vdso.lds

The native ABI vDSO linker script vdso.lds is built by preprocessing
vdso.lds.S, with the native -mabi flag passed in to get the correct ABI
definitions. Unfortunately however certain toolchains choke on -mabi=64
without a corresponding compatible -march flag, for example:

cc1: error: ‘-march=mips32r2’ is not compatible with the selected ABI
scripts/Makefile.build:338: recipe for target 'arch/mips/vdso/vdso.lds' failed

Fix this by including ccflags-vdso in the KBUILD_CPPFLAGS for vdso.lds,
which includes the appropriate -march flag.

Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: stable@vger.kernel.org # 4.4.x-
Patchwork: https://patchwork.linux-mips.org/patch/14368/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>