Re-enable LLVM LTO for ARM.

Re-enable LLVM LTO for ARM.

v8 is optimized for speed. Because GCC LTO merges flags at link time,
we disable LTO to prevent any -O2 flags from taking precedence over v8's
-Os flag. However, LLVM LTO does not work this way so we keep LTO enabled
under LLVM.

This fixes the ARM build in the cfi_vptr==1 configuration, which requires
LLVM LTO.

R=adamk@chromium.org
BUG=chromium:469376
LOG=N

Committed: https://crrev.com/fef4fab6bb2844ddc0af1c8243df81666ebd4799
Cr-Commit-Position: refs/heads/master@{#30455}

[pcc1, 2015-08-14 21:01:01]

pcc@chromium.org changed reviewers:
+ rmcilroy@chromium.org
- machenbach@chromium.org

[pcc1, 2015-08-28 18:32:03]

Ping.

[adamk, 2015-08-28 22:42:58]

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

[adamk, 2015-08-28 22:42:59]

rubberstamp lgtm as this follows similar patches elsewhere in Chromium

[adamk, 2015-08-28 22:43:04]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-08-28 22:43:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1295673002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1295673002/1

[commit-bot: I haz the power, 2015-08-28 22:46:02]

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

[commit-bot: I haz the power, 2015-08-28 22:46:02]

Try jobs failed on following builders:
  v8_presubmit on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_presubmit/builds/5359)

[pcc, 2015-08-28 22:47:20]

The CQ bit was checked by pcc@google.com

[commit-bot: I haz the power, 2015-08-28 22:47:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1295673002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1295673002/1

[commit-bot: I haz the power, 2015-08-28 23:05:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-28 23:05:48]

Patchset 1 (id:??) landed as
https://crrev.com/fef4fab6bb2844ddc0af1c8243df81666ebd4799
Cr-Commit-Position: refs/heads/master@{#30455}