Remove "singlethread": true

[lights0123]

In all the target JSON files, "singlethread": true is set. However, I don't believe that this works—interrupts can still cause problems, as an interrupt happening at the wrong place can interfere with the operation. The only operation that AVR can perform safely is an 8-bit read or write. All other operations require disabling interrupts.

[Rahix]

Right, I wasn't aware of the true meaning of the field. Thanks for pointing it out to me!

[lights0123]

For anyone coming across this in the future, I wrote a guide about what to do if you want to use the Atomic types: https://docs.rust-embedded.org/embedonomicon/custom-target.html#fill-the-target-file