More unsafe super powers

[oskgo]

The section on unsafe Rust only mentions 5 unsafe super powers, which is out of date with the 8 mentioned in the reference.

The following are new:

• Calling a safe function marked with a target_feature from a function that does not have a target_feature attribute enabling the same features (see attributes.codegen.target_feature.safety-restrictions)
• Declaring an extern block
• Applying an unsafe attribute to an item

If listing and explaining all 8 is undesirable the chapter should avoid implying that the list is exhaustive.