s2e-arm: bugfix: pass message pointer in r0 for s2earm calls

The pointer to the message for the s2e_{message,warning} calls was not
set in the 'r0' register.

Signed-off-by: Lucian Cojocar <cojocar@gmail.com>
Merge pull request #2 from cojocar/eurecom-arm-upstream

s2e-arm: bugfix: pass message pointer in r0 for s2earm calls