Fix xargs to obey POSIX's ARG_MAX restrictions.

This avoids "xargs: exec echo: Argument list too long" errors in practice.

find(1) needs to be fixed too, but that's a bit more complicated and a working
xargs provides a workaround.

Bug: http://b/65818597
Test: find /proc | strace -f -e execve ./toybox xargs echo > /dev/null