Add toybox symlinks for true & uname

This new directory will be manually managed for now, so I'm not putting
it under the existing linux-x86 directory. It will be prepended to $PATH
during the build so that the build can start adopting toybox verssions
of tools instead of the version provided by the host.

Test: m
Change-Id: Ie880fabe7c8091d2984c260390133b0c482c1a59