Also updates beta, nightly, nightlyBin, and bootstrap compilers.
Also updates the registry.
Also consolidates logic between bootstrap and nightlyBin compilers.
Also contains some miscellaneous cleanups.
Also patches firefox to build with the newer cargo
This reverts commit 9f86136cef.
Rust is nowadays required for building Firefox, so the channel updates
are blocked on this.
(It also builds fine for me.)
Newer nightlies check a new environment variable that if set will loosen
restrictions on which compiler version can be used for bootstrapping.
Upstream issue is at https://github.com/rust-lang/rust/pull/37265
The OS is identified as "10.4" rather than "osx". This commit removes
the 'rt' build script's hard coding of the "osx" suffix when building on
darwin since the cmake configuration falls back to building a 10.4
compatible libclang_rt.
