Upgrade to the latest version of cloudabi-utils, v0.15.

The most important change in this release is that it contains new
prebuilt binaries of cloudabi-reexec. These contain some important bug
fixes related to the handling of vDSOs, in a way that they provide
better forward compatibility.

In addition to that, the YAML parser in cloudabi-run follows the YAML
1.1 specification more closely.