Bump bindgen to 0.70.1

Biggest change change introduced in bindgen 0.70 is the compile-time
layout tests to to check and validate layouts. Previously these
tests were implemented as unit tests which made it somewhat
difficult to run on cross-compiled targets.

Also, drop `rustfmt_bindings()` call had been enabled by default for
a while and was removed/deprecated in bindgen 0.65.x releases.

More info:
* rust-lang/rust-bindgen#2786
* rust-lang/rust-bindgen#2787

Author: plaes