Fix semihosting with latest toolchain

Fixes issues #10:

https://github.com/ARM-software/tf-issues/issues/10

This patch changes all variables of type int to be long long to fix
the sizing and alignment problems found when building with the newer
toolchains such as Linaro GCC 13.12 or later.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>