Add notrace to lib/ucmpdi2.c

As part of the MIPS conversion to use the generic GCC library routines,
Matt Redfearn discovered that I'd missed a notrace on __ucmpdi2().  This
patch rectifies the problem.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Matt Redfearn <matt.redfearn@mips.com>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/19048/
Signed-off-by: James Hogan <jhogan@kernel.org>