`safe` keyword is not feature-gated

[compiler-errors]

In #126749, @ehuss reports:

Another issue appears that safe is not feature-gated. The following seems to build without a feature gate:

safe fn foo() {}

Originally posted by @ehuss in #126749 (comment)

Tracking:

• Tracking Issue for RFC 3484: Unsafe Extern Blocks #123743

[compiler-errors]

this regressed in #124482

[traviscross]

cc @spastorino

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high