Update conjure, webtunnel, obfs4 and snowflake to use `go mod vendor` to
fetch dependencies.

We still have some go module projects, for dependencies of
projects/ncprop279/config, which doesn't have a `go.sum` file yet.

We also update doc/MAKEFILE.txt for previous changes with some makefile
targets.

Add the needed targets needed to build privacy browser.

Updated the creation of the WASI sysroot, to be more similar to what
Mozilla does in shell scripts included in Firefox source code.
This includes the same hacks, like re-using the LLVM we already build.

We were still using lucetc, which is not needed anymore. Therefore,
this commit removes it.

`make torbrowser-testbuild-macos-x86_64`
  and
`make torbrowser-testbuild-macos-aarch64` can be used to do non-universal
testbuilds for a single arch.

Instead of having separate x86_64 and arm64 macos builds, we merge the
two builds to create an universal build, using the unify.py script from
Mozilla.

We also start renaming osx64 to macos for tor-browser-build#40067.

We don't enable aarch64 build by default yet. This will be done in a
second part (tor-browser-build#40439) where we create universal
x86-64/arm64 dmg files.

In projects/macosx-toolchain/build we also clean up the flags used to
build compiler-rt, trying to stay close to what Mozilla is doing.

We also switch back to using Ninja, to avoid an error in compiler-rt
build for aarch64:
tpo/applications/tor-browser-build#40158 (comment 2836499)

The following commands can be used to check needed toolchain updates to
build Firefox for Linux, Windows or macOS:

  make list_toolchain_updates-firefox-linux
  make list_toolchain_updates-firefox-windows
  make list_toolchain_updates-firefox-macos

Checking toolchain updates can be done with the following commands:

  make list_toolchain_updates-geckoview
  make list_toolchain_updates-application-services
  make list_toolchain_updates-android-components
  make list_toolchain_updates-fenix

This reverts commit a53e12f3.

Bug 40145: Adapt Makefile to build from a single alpha tag for all platforms again

Thanks to boklm for help with this patch.

We now generate source tarballs for firefox, torbutton and tor-launcher
during stable, alpha and nightly builds. We don't generate them with
|make testbuild|, but they can be generated with |make testbuild-src|.

However we keep setting MAR_OLD_FORMAT=1 on the stable release for now.