Ideally I'd like to soon enable sccache for rustbuild itself and some of the
stage0 tools, but for that to work we'll need some better Rust support than the
pretty old version we were previously using!

This upgrades the OSX builders to the `xcode9.3-moar` image which has 3 cores as
opposed to the 2 that our builders currently have. Should help make those OSX
builds a bit speedier!

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

All cases where it is used can be replaced by substituing run.host for
run.builder.build.build; that is its only effect. As such, it is
removable.

All uses are replaced with not accessing run.target/run.host, and
instead directly using run.builder.build.build.

Previously it was set to true when we didn't run HOSTS steps.

…directories

rustbuild: pass datadir to rust-installer

This fixes zsh completion install when $datadir != $prefix/share

…tep-1, r=alexcrichton

Remove ONLY_BUILD and ONLY_BUILD_TARGETS

Primarily removes `ONLY_BUILD` and `ONLY_BUILD_TARGETS`. These aren't actually needed in the new system since we can simply not take the relevant `host` and `target` fields if we don't want to run with them in `Step::make_run`.

This PR also includes a few other commits which generally clean up the state of rustbuild, but are not related to the `Step` changes.

Update sccache to its master branch

Ideally I'd like to soon enable sccache for rustbuild itself and some of the
stage0 tools, but for that to work we'll need some better Rust support than the
pretty old version we were previously using!

…etMisdreavus

Multiple rustdoc fixes

Fixes rust-lang#48733.

r? @QuietMisdreavus

…-Simulacrum

travis: Upgrade OSX builders

This upgrades the OSX builders to the `xcode9.3-moar` image which has 3 cores as
opposed to the 2 that our builders currently have. Should help make those OSX
builds a bit speedier!

…=QuietMisdreavus

Add missing urls

r? @QuietMisdreavus

tidy: Add a check for stray `.stderr` and `.stdout` files in UI test directories