To allow hexadecimal integers with the most significant bit set to be
passed as system call parameters, we need to broaden the set of values
acceptable to get_s32() to include u32.

If this proves confusing, then at some point in the future we could
rename this to be get_32() or something.

Change-Id: I416f80f8c135306c6b64de8b700186d0b9a005c7