Discussion: ci: architecture naming (mainly about x86-64 vs amd64 vs ??)

Main goal of this MR is let newcomers easily orient in our CI and describe things with correct names.

x86 usually stands for 32-bit (i386, i486, i686, and x86_32). Our images are 64-bit; let's name them accordingly to not confuse anyone.

i386 for cross-compilation doesn't describe current situation. Most distributions support only i486+ and, increasingly, i686. Let's call it x86_32 to be generic enough.

I kept tag names for backwards compatibility with other projects, assuming they'll be removed in 6–12 months.

Additional change to the conversion is also naming ARM_build job ARM64_build (because it's aarch64 runner).

/cc @eric

Edit: Note: I would go rather s/x86_64/x86-64/, IMHO it look better (and usually _ separates parts of the job name).