See:
- https://github.com/Homebrew/homebrew-core/issues/165388
- https://trac.macports.org/ticket/69639

Also enforce the path to x86-64 GCC.

That architecture is now essentially obsolete.

- enforce x86-64 architecture
- use “sysctl -n hw.ncpu” instead of “nproc” (which has not been installed in
  the new runner)

- drop the Julia stuff, since it is no longer used
  In particular, no longer use static linking, since builds are now only
  for testing purposes and are not shipped
- drop the Windows i686 build, since we no longer support that configuration
- bump Boost version used for Windows x86-64 build
- rename build_linux_aarch64 into build_linux_arm64 for clarity

— add “-s” to LDFLAGS instead of manually calling strip
— on GNU/Linux, do not pass “-static” to LDFLAGS. We can dynamically link
  against glibc, since in Artifacts.toml we explicitly mention that we compile
  for a glibc system (and not musl)
— use --host=i686-linux-gnu to compile for GNU/Linux 32-bit, instead of passing “-m32”
  N.B.: Under Debian, g++-i686-linux-gnu must be installed. Otherwise this will
  silently use the 64-bit compiler
— for symmetry, use --host=x86_64-linux-gnu for GNU/Linux 64-bit
— on macOS, pass “-static-libgcc” through LDFLAGS rather than CXXFLAGS; this
  requires setting CC to gcc-10
— use better names for the jobs

Otherwise the pipeline will remain blocked until someone manually starts the
deploy job (see documentation of “rules” in Gitlab CI ref.).

Ref. dynare#1728

Note that MSYS2 has now switched to zstd for compressing their packages.

Under heavy load, it can happen that no runner is available during one hour.

The cache in ~/tarballs on the runner is shared with the jobs of the main
dynare repository that create the Windows and macOS packages.

Also use better options for wget.

Windows{32,64}, Linux{32,64} and macOS{64}, also generates
informations required by the Julia package.