Remove UNREACHABLE() statements from sys-info.cc

Remove UNREACHABLE() statements from sys-info.cc

We have safe fallbacks in all cases, and it's possible to hit those
codepaths, e.g., on Linux when /proc is not mounted

BUG=v8:3544
R=ulan@chromium.org
LOG=n

Committed: https://crrev.com/96143d193143daab45b14cee840f59954d7a1036
Cr-Commit-Position: refs/heads/master@{#25940}

[ulan, 2014-12-23 14:00:24]

lgtm

[jochen (gone - plz use gerrit), 2014-12-23 14:02:22]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2014-12-23 14:03:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/818213002/1

[commit-bot: I haz the power, 2014-12-23 14:39:17]

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

[commit-bot: I haz the power, 2014-12-23 14:39:17]

Try jobs failed on following builders:
  v8_linux_arm64_rel on tryserver.v8
(http://build.chromium.org/p/tryserver.v8/builders/v8_linux_arm64_rel/builds/1910)

[jochen (gone - plz use gerrit), 2014-12-23 17:52:44]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2014-12-23 17:53:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/818213002/1

[commit-bot: I haz the power, 2014-12-23 19:25:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-23 19:25:36]

Patchset 1 (id:??) landed as
https://crrev.com/96143d193143daab45b14cee840f59954d7a1036
Cr-Commit-Position: refs/heads/master@{#25940}