Repository: rust
Issue: cargo not built when a target is specified
Labels: C-enhancement,A-cross,T-bootstrap,T-infra

`./x.py build src/tools/cargo` completes without error and without building `cargo` when a target is specified via the command line or via a `config.toml`. I'd expect that `cargo` would build the same regardless of what target is specified, but instead it does not build at all when a target is specified.

Example:
```
➜  rust git:(master) ✗ ./x.py build src/tools/cargo --target=x86_64-unknown-fuchsia
Updating submodules
    Finished dev [unoptimized] target(s) in 0.0 secs
Build completed successfully in 0:00:04
➜  rust git:(master) ✗ ./x.py build src/tools/cargo                                
Updating submodules
    Finished dev [unoptimized] target(s) in 0.0 secs
Building stage0 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage0 std from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 test artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage0 test from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage0 rustc from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 codegen artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu, llvm)
    Finished release [optimized] target(s) in 0.0 secs
Assembling stage1 compiler (x86_64-unknown-linux-gnu)
Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage1 std from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage1 test artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage1 test from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage1 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage1 rustc from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage1 codegen artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu, llvm)
    Finished release [optimized] target(s) in 0.0 secs
Assembling stage2 compiler (x86_64-unknown-linux-gnu)
Uplifting stage1 std (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage2 std from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Uplifting stage1 test (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage2 test from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Uplifting stage1 rustc (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage2 rustc from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage2 tool cargo (x86_64-unknown-linux-gnu)
   Compiling cargo v0.26.0 (file:///usr/local/google/home/cramertj/src/rust/src/tools/cargo)
    Finished release [optimized] target(s) in 13.21 secs
Build completed successfully in 0:00:23
```
cc @pylaligand