build.d: Support building with LDC on macOS arm64, by specifying -mtriple to select x86_64 target

[JohanEngelen]

To build DMD, the command becomes:
HOST_DMD="<path to ldc>/bin/ldmd2" DFLAGS="-mtriple=x86_64-apple-macos11" DMD="<path to ldc>/bin/ldmd2" make

this requires the universal macOS package of LDC. The universal package contains stdlib binaries to enable linking an x86_64 executable.

[dlang-bot]

Thanks for your pull request and interest in making D better, @JohanEngelen! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#16239"

[JohanEngelen]

CI failures on macOS 13 are known issues and unrelated to this PR. (although this PR would help in that it enables solving this known issue by switching to LDC to build on macOS 13).