Auto merge of #123347 - saethlin:only-allow-upstream-llvm-calls, r=Ni…

…lstrieb

Only allow compiler_builtins to call LLVM intrinsics, not any link_name function

This is another case of accidental reliance on `inline(never)` like I rooted out in rust-lang/rust#118770. Without this PR, attempting to build some large programs with `-Zcross-crate-inline-threshold=yes` with a sysroot also compiled with that flag will result in linker errors like this:
```
  = note: /usr/bin/ld: /tmp/cargo-installNrfN4T/x86_64-unknown-linux-gnu/release/deps/libcompiler_builtins-d2a9b69f4e45b883.rlib(compiler_builtins-d2a9b69f4e45b883.compiler_builtins.dbbc6c2ca970faa4-cgu.0.rcgu.o): in function `core::panicking::panic_fmt':
          /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:72:(.text.unlikely._ZN4core9panicking9panic_fmt17ha407cc99e97c942fE+0x31): undefined reference to `rust_begin_unwind'
```
With `-Zcross-crate-inline-threshold=yes` we can inline `panic_fmt` into `compiler_builtins`. Then we end up with a call to an upstream monomorphization, but one that has a `link_name` set. But unlike LLVM's magic intrinsic names, this link name is going to make it to the linker, and then we have a problem.

This logic looks scuffed, but also we're doing this in 4 other places. Don't know if that means it's good or bad.
https://github.com/rust-lang/rust/blob/1684a753dbca5d23b2e03568e6fbbb48eb72d0e6/compiler/rustc_codegen_cranelift/src/abi/mod.rs#L386
https://github.com/rust-lang/rust/blob/1684a753dbca5d23b2e03568e6fbbb48eb72d0e6/compiler/rustc_ast_passes/src/feature_gate.rs#L306
https://github.com/rust-lang/rust/blob/1684a753dbca5d23b2e03568e6fbbb48eb72d0e6/compiler/rustc_codegen_ssa/src/codegen_attrs.rs#L609
https://github.com/rust-lang/rust/blob/1684a753dbca5d23b2e03568e6fbbb48eb72d0e6/compiler/rustc_codegen_gcc/src/declare.rs#L170