Teach the makefile to mix in additional 'extra filename' information

[brson]

Some systems, like Gentoo, want to package multiple versions of the Rust compiler along side each other. There are several obstacles to this, but the obvious one is that the installed Rust crates need to not have conflicting names. While we have a mechanism for this --filename-extra, the extra strings appended by the current makefile are not sufficient to discriminate between arbitrary compiler revisions.

Most likely we will add a configure switch that specifies an additional string to hash into the filename extra, in addition to what we're already hashing.

re https://internals.rust-lang.org/t/perfecting-rust-packaging-the-plan/2767