Non-SFI mode: Sandbox support for NaCl async-signals.

In preparation for the async-signal support change in
https://codereview.chromium.org/1212613002, this CL allows two new
syscalls:

* clone() with CLONE_PARENT_SETTID.
* tgkill() with the current pid as tgid and SIGUSR1 as signal number.

TEST=nacl_loader_unittests
TEST=Ran bots
TEST=ARC async-signals work
BUG=nativeclient:4065
[email protected]

Review URL: https://codereview.chromium.org/1295513003

Cr-Commit-Position: refs/heads/master@{#343807}
3 files changed