x86: zero-extend 32-bit args in syscall entry instead of sign-extension

x86: zero-extend 32-bit args in syscall entry instead of sign-extension

Zero-extension is slightly more common that sign-extension:
all pointers are zero-extended, and some other params are unsigned.

Whereas signed ones (fds, pids, etc) are often treated as
_32-bit ints_ even by kernel, so just unconditionally casting
such tcp->u_arg[N] to int works.

* syscall.c (get_syscall_args): [X86] Zero-extend 32-bit args
instead of sign-extension.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>