On Apple M1 with Rosetta, how to open entire Terminal / iTerm in x86_64 architecture?

As a developer, I have experienced major issues with installing languages, libraries and development tools from the command-line as Apple M1 has migrated over to using arm64 instead of x86_64 architecture. In order to bridge the gap before all libraries and tools change over to arm64, and to avoid library incompatibilities between architectures, in many cases it may be easier for developers to just emulate x86_64 to perform their library installs.