rustdoc: unable to determine target arch from $CFG_COMPILER_HOST_TRIPLE

[ruuda]

I just built from source (b8b18aa). Then I run:

RUST_BACKTRACE=1 x86_64-unknown-linux-gnu/stage2/bin/rustdoc src/librustc/lib.rs

Output:

error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
thread 'rustc' panicked at 'unable to determine target arch from $CFG_COMPILER_HOST_TRIPLE', src/libcore/option.rs:704
stack backtrace:
   1:     0x7f5f25590760 - sys::backtrace::tracing::imp::write::hd89d31b61dd4636a0nu
   2:     0x7f5f2559740b - panicking::default_handler::_<closure>::closure.42795
   3:     0x7f5f25596f93 - panicking::default_handler::he52570f6dcb4275cEFy
   4:     0x7f5f255607b6 - sys_common::unwind::begin_unwind_inner::h3aebccca4b9bf29bygt
   5:     0x7f5f25561178 - sys_common::unwind::begin_unwind_fmt::h4813f7216103e62fEft
   6:     0x7f5f2558e371 - rust_begin_unwind
   7:     0x7f5f255d935f - panicking::panic_fmt::h370f66a0dae4fef5nYL
   8:     0x7f5f255f8e74 - option::expect_failed::h62f2fa1f9a82549ewsO
   9:     0x7f5f21287b65 - diagnostics::plugin::expand_build_diagnostic_array::h25d3f5688509a9a1Swb
  10:     0x7f5f23fed3d8 - ext::base::F.TTMacroExpander::expand::h7866968033327010450
  11:     0x7f5f2150d29d - ext::expand::expand_item_mac::h04cbae9fe49826f57A9
  12:     0x7f5f215000b1 - ext::expand::expand_annotatable::h95df9f39eb8f2216C59
  13:     0x7f5f214fd3a8 - ext::expand::expand_item::hb69cfed6380c1941ly9
  14:     0x7f5f21509155 - fold::noop_fold_mod::h15406053960004563981
  15:     0x7f5f21502dd1 - ext::expand::expand_item_underscore::hd828db04175e7b33Wy9
  16:     0x7f5f21537dfe - fold::noop_fold_item_simple::h396184506949694463
  17:     0x7f5f21537960 - fold::noop_fold_item::h17919760620970215817
  18:     0x7f5f21501469 - ext::expand::expand_annotatable::h95df9f39eb8f2216C59
  19:     0x7f5f214fd3a8 - ext::expand::expand_item::hb69cfed6380c1941ly9
  20:     0x7f5f21541672 - ext::expand::expand_crate::hc662996ec0a2f07dGta
  21:     0x7f5f24deb182 - driver::phase_2_configure_and_expand::_<closure>::closure.32909
  22:     0x7f5f24d7e8a6 - driver::phase_2_configure_and_expand::h9999dd8fbbf2e579hza
  23:     0x7f5f25aa4a1d - core::run_core::h6e8416ef3a1171b1Xdk
  24:     0x7f5f25aa0268 - sys_common::unwind::try::try_fn::h5136641819810760272
  25:     0x7f5f2558e308 - __rust_try
  26:     0x7f5f25585d6b - sys_common::unwind::try::inner_try::ha6a314e7a2e1ae926ct
  27:     0x7f5f25aa1f4f - boxed::F.FnBox<A>::call_box::h5932943521372803105
  28:     0x7f5f25595983 - sys::thread::Thread::new::thread_start::h49fd9650f679b92eoCx
  29:     0x7f5f1ea084a3 - start_thread
  30:     0x7f5f2521513c - clone
  31:                0x0 - <unknown>