MIPS: Fix sigreturn via VDSO on microMIPS kernel

MIPS: Fix sigreturn via VDSO on microMIPS kernel

In microMIPS kernels, handle_signal() sets the isa16 mode bit in the
vdso address so that the sigreturn trampolines (which are offset from
the VDSO) get executed as microMIPS.

However commit d1c3d7de99d5 ("MIPS: Initial implementation of a VDSO")
changed the offsets to come from the VDSO image, which already have the
isa16 mode bit set correctly since they're extracted from the VDSO
shared library symbol table.

Drop the isa16 mode bit handling from handle_signal() to fix sigreturn
for cores which support both microMIPS and normal MIPS. This doesn't fix
microMIPS only cores, since the VDSO is still built for normal MIPS, but
thats a separate problem.

Fixes: d1c3d7de99d5 ("MIPS: Initial implementation of a VDSO")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.4.x-
Patchwork: https://patchwork.linux-mips.org/patch/13348/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>