[arm64] Fix some signed/unsigned type inconsistencies.

[arm64] Fix some signed/unsigned type inconsistencies.

This fixes several warnings when cross-building using GCC (since r31087,
5cf1c0b).

In particular, CPURegister::code() now returns 'int', matching the other
platforms (and the coding style guide). The rest of the patch consists
of similar changes to make this work.

BUG=

Committed: https://crrev.com/29f73ad1046de941c5ae52223315bebfd6b21a2b
Cr-Commit-Position: refs/heads/master@{#31176}

[jbramley, 2015-10-08 10:02:34]

jacob.bramley@arm.com changed reviewers:
+ danno@chromium.org, ulan@chromium.org

[jbramley, 2015-10-08 10:02:45]

jacob.bramley@arm.com changed reviewers:
+ bmeurer@chromium.org

[Benedikt Meurer, 2015-10-08 11:02:56]

LGTM

[jbramley, 2015-10-08 11:08:55]

The CQ bit was checked by jacob.bramley@arm.com

[commit-bot: I haz the power, 2015-10-08 11:09:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1393043003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1393043003/1

[commit-bot: I haz the power, 2015-10-08 11:32:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-08 11:32:57]

Patchset 1 (id:??) landed as
https://crrev.com/29f73ad1046de941c5ae52223315bebfd6b21a2b
Cr-Commit-Position: refs/heads/master@{#31176}