rust @1.24.0: universal variant does not build universal

[ryandesign (Ryan Carsten Schmidt)]

$ port -v installed rust
The following ports are currently installed:
  rust @1.24.0_0+universal (active) platform='darwin 16' archs='i386 x86_64' date='2018-03-04T23:56:41-0600'
$ lipo -info /opt/local/bin/rustc
Non-fat file: /opt/local/bin/rustc is architecture: x86_64

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

Attached is a proposed solution for making rust build universal binaries.
Included is also a fix for #55956.

[ryandesign (Ryan Carsten Schmidt)]

Thanks! That's a lot more complicated than I thought it would be; sorry about that. It looks like that does work, though it's hard to tell, given how weird it is.

[ryandesign (Ryan Carsten Schmidt)]

Oh—the revision of the main port will need to be increased, since this changes what gets installed in some situations (specifically, when the universal variant was chosen). (The revision of the subport probably shouldn't be increased because I don't think its contents would change.)

It does work at least as far as:

$ rustc --version
rustc 1.24.1
$ arch -i386 rustc --version
rustc 1.24.1

Before your changes, it was:

$ rustc --version
rustc 1.24.1
$ arch -i386 rustc --version
arch: posix_spawnp: rustc: Bad CPU type in executable

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

In 02bf4b5730ae7fa7d5641089facec22b4a8b3610/macports-ports:

rust: allow universal build; UsingTheRightCompiler

Fixes #55962
Fixes #55956

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