lang/rust: Enable on FreeBSD/aarch64 + various fixes

This port now uses an unofficial bootstrap of Rust 1.17.0 and Cargo
0.18.0 for FreeBSD/aarch64. Here are my notes to create this boostrap:
https://gist.github.com/dumbbell/b587da50ef014078da9e732a4331ebad

The port's Makefile was changed to:

• allow to override the versions and directories of the bootstrap archives;
• patch the bootstrap script and manifest with those versions/directories.

Beside changes to support FreeBSD/aarch64, the port received the
following fixes:

• Pass python path to the configure script.
• Fix and simplify the extraction of rust-std bootstrap by re-using existing variables.
• Make sure tar(1) doesn't produce sparse file entries when it recreates the rust-std bootstrap because Python 2's tarfile module doesn't support them (PR 219842).

PR: 216143, 219842
Differential Revision: https://reviews.freebsd.org/D10857