Add cloudabi-clang.

[ed]

Summary

This port adds the necessary symbolic links to make Clang work as a cross compiler for CloudABI. Clang can automatically derive its target from argv[0]. We don't need to install another copy of Clang. It is sufficient to invoke it the right way.

CloudABI support is not yet part of a released version of Clang. This is why we still need to depend on llvm-devel. My intent is to stick to a fixed compiler version at one point in time.

Test Plan

The resulting compiler (x86_64-unknown-cloudabi-cc) seems to be able to compile C code properly. It also invokes the linker in the right way.

[bapt]

You should add a ONLY_FOR_ARCH to ensure this port only tries to build on amd64

[ed]

Hi Baptiste,

In D2885#56031, @bapt wrote:

You should add a ONLY_FOR_ARCH to ensure this port only tries to build on amd64

What's pretty awesome is that this isn't needed. Regardless of the architecture we're on, llvm-devel is able to target x86. By adding these symbolic links, we effectively get a cross compiler for x86-64.

The idea is that we want to add CloudABI support for other architectures. In that case we'll extend this port to simply add more symbolic links. One port that gives you a compiler for all architectures.

[bapt]

Then go ahead :)