[lld] Reject --read-workers when lld is built without thread support (#163925)

Also expand the #ifdef to remove unused code in this configuration. As
suggested in
https://github.com/llvm/llvm-project/pull/147134#issuecomment-3328612158.

I have also:
* Expanded the error message to explain why it's not allowed.
* Added a test for the error.
* Marked the original test as unsupported when threads are disabled.

Fixes issues we have had on Armv8 with threading disabled where this
test would crash every so often.

This change will hopefully be superseded by #157917, but that has been
in review a long time and I want to make the bot stable again.

I could just disable the test, but I'd like lld to function properly in
general in the meantime too.

Co-authored-by: John Holdsworth <github@johnholdsworth.com>

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