This fixes several warnings when cross-building using GCC (since r31087,
5cf1c0bc).

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=

Review URL: https://codereview.chromium.org/1393043003

Cr-Commit-Position: refs/heads/master@{#31176}