`-Zdefault-hidden-visibility` causes linking errors due to intrinsic-related calls

-Zdefault-hidden-visibility should affect symbols defined in a crate. References to externally-defined symbols should not have hidden visibility.

Certain intrinsics such as compare_bytes may emit calls to libc.so functions (e.g. memcmp). When -Zdefault-hidden-visibility is used, these symbol references are hidden. This leads to linking errors since an undefined hidden symbol cannot be resolved to a symbol in a dynamic library.

Repro:

#![feature(core_intrinsics)]

fn main() {
    let a = 0u8;
    let b = 0u8;

    println!("{}", unsafe {
        std::intrinsics::compare_bytes(&a as *const _, &b as *const _, 1)
    });
}

error:

note: /usr/bin/ld: .../rust/build/x86_64-unknown-linux-gnu/test/ui/default-hidden-visibility-intrinsic/default-hidden-visibility-intrinsic.default_hidden_visibility_intrinsic.adfc02789a1ff675-cgu.0.rcgu.o: in function `default_hidden_visibility_intrinsic::main':
           default_hidden_visibility_intrinsic.adfc02789a1ff675-cgu.0:(.text._ZN35default_hidden_visibility_intrinsic4main17hf16163bc798359ebE+0x1e): undefined reference to `memcmp'
           /usr/bin/ld: .../rust/build/x86_64-unknown-linux-gnu/test/ui/default-hidden-visibility-intrinsic/default-hidden-visibility-intrinsic: hidden symbol `memcmp' isn't defined
           /usr/bin/ld: final link failed: bad value
           collect2: error: ld returned 1 exit status

The flag was added in https://github.com/rust-lang/rust/pull/118417; MCP https://github.com/rust-lang/compiler-team/issues/656

While this flag isn't really intended for use on bin crates, I suspect crates downstream of an rlib or staticlib would have the same errors.

Start with the provided compare_bytes reproduction and the regression-test path test/ui/default-hidden-visibility-intrinsic/default-hidden-visibility-intrinsic. Trace how -Zdefault-hidden-visibility is applied to intrinsic-emitted references such as memcmp. Done means externally defined symbols retain resolvable visibility and the reproduction links successfully with a regression test.