syntax: implement cfg!() which evaluates to true/false where #[cfg] would keep/remove.

Example:

   if cfg!(test) {
      calculation_to_run_only_when_testing();
   }
Add a boatload of Linux x86/x86-64/arm errnos
Relax some atomic barriers. Loosen up all that tension. There, doesn't that feel good?
Document task killing design and relaxed barrier rationale.
Make a forgotten assert in comm be cfg(test)-dependent
Temporary workaround to prevent taskgroup cleanup code from failing without an exception handler.
Make crate tests depend on their extern crates.  Reorder TEST_HOST_CRATES in hopes of exposing future instances of this bug.
std: Implement SingleThreaded spawn mode for newsched
auto merge of #8188 : huonw/rust/cfg-macro, r=pcwalton

Example:

    if cfg!(test) {
       calculation_to_run_only_when_testing();
    }

Closes #8130.
auto merge of #8193 : cmr/rust/linux-errno, r=graydon
auto merge of #8195 : bblum/rust/task-cleanup, r=brson

In the first commit it is obvious why some of the barriers can be changed to ```Relaxed```, but it is not as obvious for the once I changed in ```kill.rs```. The rationale for those is documented as part of the documenting commit.

Also the last commit is a temporary hack to prevent kill signals from being received in taskgroup cleanup code, which could be fixed in a more principled way once the old runtime is gone.
auto merge of #8202 : pnkfelix/rust/fsk-issue8056-make-crates-must-dep-on-subcrates, r=graydon

Reorder TEST_HOST_CRATES in hopes of exposing future instances of this bug.

Fix #8056
auto merge of #8221 : brson/rust/single-threaded, r=graydon

This is the last major runtime feature needed for the transition to the new scheduler.
Add trait name to conflicting implementation error message
Add backticks around trait name in conflicting implementations error message