crash-reporter: Add MIPS support for KernelCollector.

BUG=chromium:396465
TEST=`FEATURES=test emerge-x86-generic crash-reporter`
TEST=`FEATURES=test emerge-amd64-generic crash-reporter`
TEST=`emerge-arm-generic crash-reporter`
TEST=`emerge-mipsel-o32-generic-generic crash-reporter`

Change-Id: I270b56d3f0b6941c97ebec6e736230701e5c3e1f
Reviewed-on: https://chromium-review.googlesource.com/209485


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>