Do not panic in Termination impl on closed stderr

Repro:

    #![feature(backtrace)]

    use std::backtrace::Backtrace;
    use std::io::{self, Write as _};
    use std::panic::{self, PanicInfo};

    #[derive(Debug)]
    pub struct Error;

    fn panic_hook(panic_info: &PanicInfo) {
        let backtrace = Backtrace::force_capture();
        let _ = write!(io::stdout(), "{}\n{}", panic_info, backtrace);
    }

    fn main() -> Result<(), Error> {
        panic::set_hook(Box::new(panic_hook));
        let stderr = io::stderr();
        let mut stderr = stderr.lock();
        while stderr.write_all(b".\n").is_ok() {}
        Err(Error)
    }

Before:

    $ RUST_BACKTRACE=1 target/debug/testing 3>&2 2>&1 1>&3 | head
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    panicked at 'failed printing to stderr: Broken pipe (os error 32)', library/std/src/io/stdio.rs:1016:9
       0: testing::panic_hook
                 at ./src/main.rs:11:21
       1: core::ops::function::Fn::call
                 at /git/rust/library/core/src/ops/function.rs:77:5
       2: std::panicking::rust_panic_with_hook
       3: std::panicking::begin_panic_handler::{{closure}}
       4: std::sys_common::backtrace::__rust_end_short_backtrace
       5: rust_begin_unwind
       6: core::panicking::panic_fmt
       7: std::io::stdio::_eprint
       8: <core::result::Result<!,E> as std::process::Termination>::report
                 at /git/rust/library/std/src/process.rs:2164:9
       9: <core::result::Result<(),E> as std::process::Termination>::report
                 at /git/rust/library/std/src/process.rs:2148:25
      10: std::rt::lang_start::{{closure}}
                 at /git/rust/library/std/src/rt.rs:145:18
      11: std::rt::lang_start_internal
      12: std::rt::lang_start
                 at /git/rust/library/std/src/rt.rs:144:17
      13: main
      14: __libc_start_main
                 at /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
      15: _start

After:

    $ RUST_BACKTRACE=1 target/debug/testing 3>&2 2>&1 1>&3 | head
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0 files changed, 0 insertions, 0 deletions