lang/rust: Update to 1.26.0

The announcement is available on Rust blog:
https://blog.rust-lang.org/2018/05/10/Rust-1.26.html

Rust 1.25.0, used as the bootstrap compiler, contains the patch from
bdrewery@ to use the compat (non-ino64) symbols on FreeBSD 12-CURRENT.
Therefore, we don't need old_fstat.c and the associated machinery in
the Makefile anymore.