LTO turns weak linkage into a miscompile (identified via `gettid` segfault on x86_64-unknown-linux-musl)

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This is a very strange one. Since 289fe36d373 (I bisected; cc @tgross35), my program that calls libc's gettid segfaults when built for x86_64-unknown-linux-musl. However, I can only reproduce this when the call to gettid is in a library crate, and only when it has a certain name, and only when not using -C target-feature=-crt-static, only when using LTO.

Code

dep.rs

unsafe extern "C" {
    safe fn gettid() -> i32;
}

pub fn print_tid() {
    println!("{}", gettid());
}

main.rs

fn main() {
    vmm::print_tid();
}

Reproduction commands

rustc --crate-name vmm --edition=2024 dep.rs --crate-type lib -C opt-level=3 -C linker-plugin-lto --target x86_64-unknown-linux-musl -C linker=/nix/store/nz3n5wy8zadq73kkql2xn485l7xdmv46-x86_64-unknown-linux-musl-gcc-wrapper-15.2.0/bin/x86_64-unknown-linux-musl-cc
rustc --edition=2024 main.rs --crate-type bin -C opt-level=3 -C lto --target x86_64-unknown-linux-musl -C linker=/nix/store/nz3n5wy8zadq73kkql2xn485l7xdmv46-x86_64-unknown-linux-musl-gcc-wrapper-15.2.0/bin/x86_64-unknown-linux-musl-cc --extern vmm=libvmm.rlib
./main

Happy to provide LLVM IR or anything else that helps!

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

Start by running the supplied dep.rs and main.rs reproduction commands for x86_64-unknown-linux-musl, comparing LTO with and without -C target-feature=-crt-static. Inspect regression commit 289fe36d373 and the generated LLVM IR around the gettid call and weak linkage. Done means the reproducer no longer segfaults under LTO and the regression is covered by a compiler test.