Fix semihosting with latest toolchain

Fixes issues #10:

https://github.com/ARM-software/tf-issues/issues/10

This patch changes all/most variables of type int to be size_t 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>