detect: Use weakref instead of dlsym - #243

$ # With #[linkage = "extern_weak"] static my_extern: Option<unsafe extern "C" fn() -> i32>;
$ rustc --crate-name vmm --crate-type lib -C opt-level=3 dep.rs -g; rustc main.rs -C opt-level=3 -C lto --extern vmm=libvmm.rlib -g; ./main
zsh: segmentation fault  ./main

$ # With weakref!(my_extern, Option<unsafe extern "C" fn() -> i32>);
$ rustc --crate-name vmm --crate-type lib -C opt-level=3 dep.rs -g; rustc main.rs -C opt-level=3 -C lto --extern vmm=libvmm.rlib -g; ./main
error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "-Wl,--fix-cortex-a53-843419" "<1 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "<sysroot>/lib/rustlib/aarch64-unknown-linux-gnu/lib/libcompiler_builtins-*.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/mnt/mac/Users/taiki/projects/tmp/repro/rustch92wgI/raw-dylibs" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/aarch64-unknown-linux-gnu/lib" "-o" "main" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-Wl,-O1" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: /usr/bin/ld: main.main.624f8795ab9c9f81-cgu.0.rcgu.o:(.data.rel.ro.__portable_atomic__weak_my_extern+0x0): undefined reference to `my_extern'
          /usr/bin/ld: main.main.624f8795ab9c9f81-cgu.0.rcgu.o: in function `main::main':
          /mnt/mac/Users/taiki/projects/tmp/repro/dep.rs:7:(.text._RNvCs8riVLv9cHGn_4main4main+0x20): undefined reference to `my_extern'
          collect2: error: ld returned 1 exit status

  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link

$ rustc -vV
rustc 1.99.0-nightly (84b36a78a 2026-08-06)
binary: rustc
commit-hash: 84b36a78a28a63f134171c670be1932ffa2485f8
commit-date: 2026-08-06
host: aarch64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 23.1.0