tlsdate-seccomp: Limit arguments for mmap and mprotect.

This eliminates some of the ways to get writable executable memory.

BUG=b:172212144
TEST=CQ passes

Change-Id: I148f40e6f7c6f481b8d4e24e7c80389056e7476a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/tlsdate/+/3265565
Tested-by: Allen Webb <allenwebb@google.com>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Allen Webb <allenwebb@google.com>
4 files changed