Use Rust 1.34 for Tor Browser 9

https://bugzilla.mozilla.org/show_bug.cgi?id=1536543 set the Rust requirement to 1.34 and we should follow that if there are no strong reasons against that. While adapting our Rust part we should/could consider the following tasks:

1. We don't need to build a newer Rust anymore just for the tor nightlies
2. Given our work on the mingw-w64/clang toolchain for legacy/trac#28328 (moved), which requites a pretty new toolchain, we could test using the llvm we already build for that one to copmile Rust instead of building an own llvm during Rust build
3. We need to switch the target for armv7 to thumbv7neon-linux-androidabi (see: https://bugzilla.mozilla.org/show_bug.cgi?id=1521734 for more details).