seccomp: use names for PROT_EXEC and PROT_WRITE

It's much easier to read the policy if you don't have to work out that
0xfffffffb is the bitwise NOT of PROT_EXEC.

BUG=none
TEST=none

Change-Id: I0f4bb4c3cff8abd466afc735a202e279b36288af
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/touch_updater/+/3945148
Reviewed-by: Allen Webb <allenwebb@google.com>
Tested-by: Harry Cutts <hcutts@chromium.org>
Commit-Queue: Harry Cutts <hcutts@chromium.org>
11 files changed