Auto merge of #1243 - alexcrichton:issue-1239, r=brson

Previously a build script would assign itself to the platform requirement for
the first target of a package, but it's more correct to consider all targets for
platform requirements as there may be both a plugin and target requirement.

Closes #1236

diff --cc src/cargo/ops/cargo_rustc/context.rs
Simple merge

diff --cc src/cargo/ops/cargo_rustc/mod.rs
Simple merge

diff --cc tests/test_cargo_cross_compile.rs
Simple merge