[build-script] There can only be one host target

[modocache]

What's in this pull request?

build-script-impl currently maintains a list of NATIVE_TOOLS_DEPLOYMENT_TARGETS -- host machine targets, for which the resulting binaries can be run on the current machine.

However, there is only ever one host machine. This commit:

• Changes the NATIVE_TOOLS_DEPLOYMENT_TARGETS list parameter into a single string parameter, HOST_TARGET.
• Promotes the logic to detect the host target to Python, and places it in the swift_build_support module.
• Removes the hard-coded "macosx_x86_64" path to the LLVM and Clang TableGen -- thereby unblocking future work to make cross-compilation possible on platforms other than OS X.
• Also promotes cross-compilation target validation to Python, placing it in the swift_build_support module.

Why merge this pull request?

1. It continues work on SR-237 by pulling more of the build-script-impl logic out of the shellscript and into Python.
2. It removes hardcoded "macosx-x86-64" parameters from the build script. This is useful if we ever want the build script to allow cross-compilation from host machines other than OS X--which I think is indeed something we do want in the near future.

What downsides are there to merging this pull request?

• It changes NATIVE_TOOLS_DEPLOYMENT_TARGETS from a list to a single element. I don't think there is any need multiple host targets, but I may be misunderstanding something.

[gparker42]

The only case for multiple host targets that I can think of is simultaneous 32- and 64-bit support. I don't think we use that anywhere today (in particular, Swift doesn't support 32-bit OS X). If we do need it we can probably say 64-bit is native and 32-bit is cross or something.

[modocache]

The only case for multiple host targets that I can think of is simultaneous 32- and 64-bit support.

Aha, of course! Thanks for pointing that out.

If we do need it we can probably say 64-bit is native and 32-bit is cross or something.

Agreed. I'm hoping to expand the concept of "compilation targets" in upcoming pull requests, particularly to support cross-compilation from OS X to Linux ARM.

[gribozavr]

Right, parallel 32 and 64 bit Linux builds was the motivation for using a list here.

[modocache]

I also noticed this hard-coded path, so I amended the commit to remove it as well.

[modocache]

@gribozavr Do you think this is a reasonable approach for now? Or would you prefer something that would make it easier to add parallel 32- and 64-bit builds in the future?

[modocache]

@gribozavr Friendly ping! I'd greatly appreciate feedback on what I could change here, or whether this isn't something you'd be interested in merging. 🙇

[tkremenek]

@gribozavr Any other feedback? (Aside from this change no longer merges cleanly)

[gribozavr]

@modocache Sorry for the delayed response! The patch LGTM, please rebase to the current master, and I'll merge, thank you!

[modocache]

@gribozavr Not at all! I'm grateful for the review. I've updated and did another local test run; this appears to be working fine and is ready to be merged.

[gribozavr]

Thank you!