Purge TargetPlatform_NAME and cTargetPlatformString

This is me ripping out target hard-coding, leading to !905 (closed) then !892 (closed).

A notability side effect of this is error messages cannot mention the target platform; even if we loaded it from settings file, that wouldn't be available for all panics.

I'm OK with this; dynflag/settings is already an ambient authority problem where code shouldn't be aware of configuration that doesn't already effect it. Surely there are target-agnostic panics in the GHC code-base.

CC @angerman

Progress towards #16503 (closed).

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.