Cleanup logic for deciding when to install linux sysroot

Cleanup logic for deciding when to install linux sysroot

The only semantic change here is that the sysroot will
be installed for ia32 cross builds by default, like
it is for arm and mips already.

BUG=504446
Committed: https://crrev.com/dd181aed3e76b963564500fe6f28731c9e33700c
Cr-Commit-Position: refs/heads/master@{#339343}

[Sam Clegg, 2015-07-17 20:08:05]

sbc@chromium.org changed reviewers:
+ thestig@chromium.org

[Lei Zhang, 2015-07-17 21:12:40]

lgtm

[Sam Clegg, 2015-07-17 22:03:17]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-07-17 22:04:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1240123004/1

[commit-bot: I haz the power, 2015-07-17 22:13:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-17 22:15:51]

Patchset 1 (id:??) landed as
https://crrev.com/dd181aed3e76b963564500fe6f28731c9e33700c
Cr-Commit-Position: refs/heads/master@{#339343}