Revert "Use the libc clone wrapper in sys_clone."

Revert "Use the libc clone wrapper in sys_clone."

This reverts commit d8a593bceaf0a4b38d06a8c13b948202d205f1b6.
Reason for revert: see crbug.com/442817

> Previously, we directly invoked the syscall, which would not update
> libc's PID cache in the child. Although the libc wrapper function
> updates the PID cache, it unfortunately requires that the child run on a
> different stack, even if CLONE_VM is not specified. We work around this
> by briefly switching stacks in the child, then using longjmp to switch
> back. This gives us a version of clone with fork-like behavior, which is
> what we need for starting processes in new namespaces.
>
> BUG=312380
> Committed: https://crrev.com/d8a593bceaf0a4b38d06a8c13b948202d205f1b6
> Cr-Commit-Position: refs/heads/master@{#308510}

BUG=312380, 442817
TBR=rickyz@chromium.org

Committed: https://crrev.com/ddc896504d83fa943ede4732ed2c922c7e494154
Cr-Commit-Position: refs/heads/master@{#308636}

[oshima, 2014-12-16 18:34:29]

The CQ bit was checked by oshima@chromium.org

[commit-bot: I haz the power, 2014-12-16 18:35:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/807213002/1

[rickyz (no longer on Chrome), 2014-12-16 18:37:51]

lgtm

[commit-bot: I haz the power, 2014-12-16 19:34:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-16 19:35:12]

Patchset 1 (id:??) landed as
https://crrev.com/ddc896504d83fa943ede4732ed2c922c7e494154
Cr-Commit-Position: refs/heads/master@{#308636}