linker: Report linker flavors incompatible with the current target
The linker flavor is checked for target compatibility even if linker is never used (e.g. we are producing a rlib).
If it causes trouble, we can move the check to `link.rs` so it will run if the linker (flavor) is actually used.
And also feature gate explicitly specifying linker flavors for tier 3 targets.
The next step is supporting all the internal linker flavors in user-visible interfaces (command line and json).
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