Rollup merge of #159112 - sbogomolov:freebsd-static-environ, r=clarfonthey

Fix segfault in env access in statically linked FreeBSD binaries

On FreeBSD `environ` is provided by the startup object linked into every executable (crt1.o) rather than by libc.so, so rust-lang/rust#153718 switched std to a dlsym lookup to keep shared libraries linkable with `-Wl,--no-undefined` (rust-lang/rust#153451). But dlsym needs a dynamic symbol table, which statically linked executables do not have: it returns null, and both `env::vars_os` and the `Command` spawn paths dereference it (rust-lang/rust#158939).

Resolve `environ` through a weak reference first: crt1.o defines it in every executable, statically linked ones included, and a weak undefined symbol does not break shared library links. Fall back to dlsym only when the weak reference is null, i.e. inside a shared library, where the dynamic lookup is available. Also stop dereferencing a null table pointer in `env()`, so an unresolvable `environ` degrades to an empty environment instead of a crash.

The `extern_weak` reference is the same mechanism std already uses for `__dso_handle` and `__cxa_thread_atexit_impl` in `sys/thread_local/destructors/linux_like.rs`.

## Testing

CI cannot execute FreeBSD tests (the target is cross-compiled only), so:

- `./x check library/std --target x86_64-unknown-freebsd` and a host check pass.
- The reproduction from rust-lang/rust#158939, built with `-C target-feature=+crt-static`, was executed on FreeBSD 14.4-RELEASE (amd64): built with stock 1.96.0 it segfaults on `vars_os()` (SIGSEGV, exit 139); built against std from this branch it iterates the environment, spawns a child with an inherited environment, and exits cleanly.
- Downstream, the crash was originally isolated and worked around in a production daemon whose CI runs the exact affected configuration (static, LTO) on a real FreeBSD kernel per commit; that harness stands ready to validate a nightly containing this fix.

This is a regression from stable to stable (1.95 -> 1.96) and may be worth a beta backport.

Fixes rust-lang/rust#158939