semihosting

Another cleanup patch tripped over the fact we weren't being careful
in our casting. Fix the casts, allow for a non-const and switch from
g_realloc to g_renew.

The whole semihosting argument handling could do with some tests
though.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220315121251.2280317-8-alex.bennee@linaro.org>