ping me when target ABI checks are being changed

[RalfJung]

The first commit moves the target checks into a separate file, so that the second commit can set up triagebot to ping me when that file gets changed.

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

r? @chenyukang

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[RalfJung]

I am not sure why this message is printed even when we just change the infrastructure around targets, but do not change any actual target. I think checking just the targets folder is enough.
Cc @ehuss
(#103605)

View changes since the review

[chenyukang]

@bors r+ rollup

[rust-bors]

📌 Commit 41e670d has been approved by chenyukang

It is now in the queue for this repository.