Start using the toybox prebuilt

Prepend the toybox symlink directory to $PATH, and stop generating
symlinks into out/.path for tools used from toybox.

We don't (yet?) have toybox prebuilts for darwin, so apply the above only to Linux, and preserve existing behavior on Darwin.

Test: check the build banner for uname results
Test: m
Change-Id: I37fc380381e65a628cdc131d462fd4441eacfe9d