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

Required moving all fulldeps tests depending on `rand` to different locations as
now there's multiple `rand` crates that can't be implicitly linked against.

Pulls in a redesigned `std::simd` module as well as a replacement for the
`is_target_feature_detected!` macro

rustbuild: pass datadir to rust-installer

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

…imulacrum

Update Cargo submodule

Hopefully a routine update...

…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.

…-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

Update stdsimd module

Pulls in a redesigned `std::simd` module as well as a replacement for the
`is_target_feature_detected!` macro