devel/cargo: Update to 0.15.0, enable on i386

The new version requires cargo(1) to be available. Therefore, we let the
Ports system to extract the bootstrap cargo (it was done by the Cargo
build system before) and set a configure flag to point to it.

We need to set OPENSSL_DIR as a Make environment variable because the
openssl-sys crate's build.rs doesn't know about FreeBSD.

Finally, enable devel/cargo on i386. A bootstrap cargo executable is
available for FreeBSD/i386 since last October but I missed it. We can
now use it to build a port/package for i386.

PR: 216143
Approved by: bapt (mentor)
Differential Revision: https://reviews.freebsd.org/D9273