Auto merge of #97657 - Urgau:check-cfg-many-mut, r=oli-obk

Use get_many_mut to reduce the cost of setting up check cfg values

This PR use the newly added [`get_many_mut`](rust-lang/rust#97601) function in [`HashMap`](https://doc.rust-lang.org/nightly/std/collections/hash_map/struct.HashMap.html#method.get_many_mut) to reduce the cost of setting up the initial check cfg values.

cc `@petrochenkov`

Author: bors