Add missing num_internals feature gate to coretests/benches

[bjorn3]

This will allow getting rid of the patch at https://github.com/rust-lang/rustc_codegen_cranelift/blob/main/patches/0030-sysroot_tests-Add-missing-feature-gate.patch

[rustbot]

r? @jieyouxu

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[lqd]

2 questions:

• the patch has two changes, but this PR has one?
• if this feature gate is only used in the cg_clif benches/tests, we should say as such in a comment, otherwise someone could remove it as being unused, right?

[bjorn3]

the patch has two changes, but this PR has one?

The other change was already done upstream by someone else.

if this feature gate is only used in the cg_clif benches/tests, we should say as such in a comment, otherwise someone could remove it as being unused, right?

It is not. I suspect the reason it doesn't fail in Rust's CI is benches not being tested at all in CI.

[jieyouxu]

r? libs