detect: Use weakref instead of dlsym

[taiki-e]

This is similar to dlsym and unstable #[linkage = "extern_weak"], but async-signal-safe unlike dlsym and works on stable and sound with fat LTO unlike extern_weak.

When statically linked, weakref may resolve to null even if the symbol is actually available (as with dlsym and extern_weak).
However, unlike dlsym it works if there are other strong references to the same symbol, so this PR enables detection at all times for targets where it was previously enabled only with dynamic linking.

Unlike extern_weak, this can be soundly used with "fat LTO + other strong reference + symbol resolved to null" case: rust-lang/rust#154439

With rust-lang/rust#154439 (comment) example on aarch64-unknown-linux-gnu:

(A weak reference is promoted to a strong reference as a strong reference already exists in deps.rs, and since there is no my_extern, it is correct that a link error occurs in this example.)

$ # 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

See comments on weakref! macro added to src/utils.rs for more details.

Refs: https://sourceware.org/binutils/docs/as/Weakref.html