minijail: Fix architecture validation for 32-bit MIPS platforms.

minijail did not set ARCH_NR correctly such that it always assumed any
MIPS platform to be 64-bit, which caused the architecture validation in
the seccomp filter to fail on a 32-bit MIPS platform.

BUG=chromium:416734
TEST=Tested a seccomp filter via minijail on a 32-bit MIPS platform.

Change-Id: I26489f0b80e48c30ee39d256218b48f927cd74b4
Reviewed-on: https://chromium-review.googlesource.com/219434
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
1 file changed