PPC64: Adjust simulator stack safety margin.

PPC64: Adjust simulator stack safety margin.

This fixes a failure in test/mjsunit/regress/regress-crbug-491062.js.

R=dstence@us.ibm.com, michael_dawson@ca.ibm.com, svenpanne@chromium.org
BUG=

Committed: https://crrev.com/440a1c7a36f7f3d82a3e5c648691c6a3f9780329
Cr-Commit-Position: refs/heads/master@{#28924}

[dstence, 2015-06-09 19:27:51]

lgtm

[MTBrandyberry, 2015-06-09 19:29:22]

The CQ bit was checked by mbrandy@us.ibm.com

[commit-bot: I haz the power, 2015-06-09 19:29:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174693002/1

[commit-bot: I haz the power, 2015-06-09 19:32:08]

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

[commit-bot: I haz the power, 2015-06-09 19:32:09]

Try jobs failed on following builders:
  v8_presubmit on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_presubmit/builds/3271)

[MTBrandyberry, 2015-06-09 19:33:35]

mbrandy@us.ibm.com changed reviewers:
+ svenpanne@chromium.org

[dstence, 2015-06-10 13:24:43]

lgtm

[MTBrandyberry, 2015-06-10 15:53:09]

Still need approval from Sven for the flag-definitions.h change.

[Sven Panne, 2015-06-11 06:41:54]

The CQ bit was checked by svenpanne@chromium.org

[Sven Panne, 2015-06-11 06:41:54]

lgtm

[commit-bot: I haz the power, 2015-06-11 06:42:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174693002/20001

[commit-bot: I haz the power, 2015-06-11 07:07:43]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-11 07:08:02]

Patchset 2 (id:??) landed as
https://crrev.com/440a1c7a36f7f3d82a3e5c648691c6a3f9780329
Cr-Commit-Position: refs/heads/master@{#28924}