Revert of Non-SFI mode: Add Linux asynchronous signal support

Revert of Non-SFI mode: Add Linux asynchronous signal support (patchset #12 id:220001 of https://codereview.chromium.org/1212613002/ )

Reason for revert:
test_async_safe_signal seems to be broken on x86_32 bots, but not reproducible on trybots or local machine.

Original issue's description:
> Non-SFI mode: Add Linux asynchronous signal support
> 
> This change adds a mechanism to both receive and send signals to other
> threads in the process.  A single signal is implemented with no option
> to mask it, but user code can use this to multiplex multiple signals and
> implement something that looks more like POSIX.
> 
> Enabling signals will require allowing the tgkill system call (with the
> current pid and SIGUSR1 as signum) and adding the CLONE_PARENT_SETTID
> flag to clone.
> 
> * Refactored some functions common to both signal and exception
>   handling from src/nonsfi/linux/irt_exception_handling.c to
>   src/nonsfi/linux/irt_signal_handling.c.  That way, other code
>   can still call |nonsfi_initialize_signal_handler| and it will
>   install both.
> * Added a test for user async-signals.
> * Added documentation for this feature in
>   documentation/nonsfi_mode_async_signals.txt
> 
> BUG=https://code.google.com/p/nativeclient/issues/detail?id=4065
> TEST=./scons run_user_async_signal_test_irt bitcode=1 nonsfi_nacl=1 \
>      --mode=nacl,dbg-host,nacl_irt_test [platform=arm]
> TEST=git try
> 
> Committed: https://chromium.googlesource.com/native_client/src/native_client/+/6a0ce065f481a3eb0bcd63ed62755d82d339c617

TBR=mseaborn@chromium.org,uekawa@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=https://code.google.com/p/nativeclient/issues/detail?id=4065

Committed: https://chromium.googlesource.com/native_client/src/native_client/+/e73a2db66b7730fb7863b53839d071f6372fd71d

[Luis Héctor Chávez, 2015-08-19 22:13:46]

Created Revert of Non-SFI mode: Add Linux asynchronous signal support

[commit-bot: I haz the power, 2015-08-19 22:13:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1300883003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1300883003/1

[commit-bot: I haz the power, 2015-08-19 22:13:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-19 22:13:55]

No L-G-T-M from a valid reviewer yet. Only full committers are accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Derek Schuff, 2015-08-19 22:15:03]

dschuff@chromium.org changed reviewers:
+ dschuff@chromium.org

[Derek Schuff, 2015-08-19 22:15:04]

lgtm, thanks

[Derek Schuff, 2015-08-19 22:17:08]

The CQ bit was checked by dschuff@chromium.org

[commit-bot: I haz the power, 2015-08-19 22:17:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1300883003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1300883003/1

[commit-bot: I haz the power, 2015-08-19 22:17:35]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/native_client/src/native_client/+/e73a2db66...