Auto merge of #32549 - respeccing:rust_backtrace_disabled, r=alexcrichton

Auto merge of #32549 - respeccing:rust_backtrace_disabled, r=alexcrichton

allow RUST_BACKTRACE=0 to act as if unset

**UPDATE:** `RUST_BACKTRACE=0` to act as if the env. var is unset! (now `0` is what `disabled` was for, below)

When RUST_BACKTRACE is set to "disabled" then this acts as if the env. var is unset. So, either make sure `RUST_BACKTRACE` is not set OR set it to `disabled` to achieve the same effect.

Sample usage:

```bash
$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && RUST_BACKTRACE=disabled /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.

$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && RUST_BACKTRACE=1 /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
stack backtrace:
   1:     0x55709e8148c0 - sys::backtrace::tracing::imp::write::h140f24a0cfc189b98Ru
   2:     0x55709e816a5b - panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::closure.45165
   3:     0x55709e8166e8 - panicking::default_hook::hed419823688cb82aXoA
   4:     0x55709e810fff - sys_common::unwind::begin_unwind_inner::hbb9642f6e212d56fmHt
   5:     0x55709e810513 - sys_common::unwind::begin_unwind::h16232867470678019594
   6:     0x55709e810489 - main::hb524f9576270962feaa
   7:     0x55709e816314 - sys_common::unwind::try::try_fn::h1274188004693518534
   8:     0x55709e813dfb - __rust_try
   9:     0x55709e815dab - rt::lang_start::h712b1cd650781872ahA
  10:     0x55709e810679 - main
  11:     0x7efd1026859f - __libc_start_main
  12:     0x55709e810348 - _start
  13:                0x0 - <unknown>
```

Some programs(eg. [vim's syntactic](https://github.com/scrooloose/syntastic) used by [rust.vim](https://github.com/rust-lang/rust.vim)) cannot unset the env. var RUST_BACKTRACE if it's already set(eg. in .bashrc) but [they can set it to some value](https://github.com/respeccing/gentooskyline/blob/cb5533e1598f871d3fdf7c3d8248ce767b5b9360/system/Z575/OSes/gentoo/on_baremetal/filesystem_now/gentoo/home/zazdxscf/build/1nonpkgs/rust.vim/upd#L17), and I needed to ensure the env. var is unset in order to avoid this issue: https://github.com/rust-lang/rust/issues/29293

**EDIT:** Sample usage 2:

```bash
$ export RUST_BACKTRACE=1

$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
stack backtrace:
   1:     0x55c2696738c0 - sys::backtrace::tracing::imp::write::h140f24a0cfc189b98Ru
   2:     0x55c269675a5b - panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::closure.45165
   3:     0x55c2696756e8 - panicking::default_hook::hed419823688cb82aXoA
   4:     0x55c26966ffff - sys_common::unwind::begin_unwind_inner::hbb9642f6e212d56fmHt
   5:     0x55c26966f513 - sys_common::unwind::begin_unwind::h16023941661074805588
   6:     0x55c26966f489 - main::hb524f9576270962feaa
   7:     0x55c269675314 - sys_common::unwind::try::try_fn::h1274188004693518534
   8:     0x55c269672dfb - __rust_try
   9:     0x55c269674dab - rt::lang_start::h712b1cd650781872ahA
  10:     0x55c26966f679 - main
  11:     0x7f593d58459f - __libc_start_main
  12:     0x55c26966f348 - _start
  13:                0x0 - <unknown>

$ rustc -o /tmp/a.out -- <(echo 'fn main(){ panic!() }') && RUST_BACKTRACE=disabled /tmp/a.out
!! executing '/home/zazdxscf/build/1nonpkgs/rust/rust//x86_64-unknown-linux-gnu/stage2/bin//rustc' with args: '-o /tmp/a.out -- /dev/fd/63'
thread '<main>' panicked at 'explicit panic', /dev/fd/63:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.

```