Rollup merge of #119588 - Nemo157:i586-netbsd-tier-3, r=Nilstrieb

Move `i586-unknown-netbsd` from tier 2 to tier 3 platform support table

It appears it was intended to be tier 3, but was accidentally added to tier 2. Based on inspecting the PR adding it the table rust-lang/rust#117170 and the fact that it is not built in CI which is one of the tier 2 requirements.

cc ````@he32````

r? ````@Nilstrieb````