Add support for '-platformlib' option. Fixes #2031

[dbankov-vmware]

This change adds the newly introduced LDC linker option '-platformlib'
to the isLinkeDFlag(...) function in ldc.d so the option to be passed to
LDC when invoked by DUB for linking.

[dlang-bot]

Thanks for your pull request and interest in making D better, @dbankov-vmware! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

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[Geod24]

Please give a proper title and description to both PR and commits.

Remember that:

1. Many people might "Watch" this repository, and a title like "Fix issue 123" means that they have to open the notification to see if that's something they are interested / competent in.
   Now multiply that by all the watchers and all the PRs that are submitted to this repository, and you quickly see how saving a minute on properly explaining changes wastes time for everyone else.

2. Not only the Github notification matters, but the git history as well. Contributors routinely use git blame or other means of displaying the history while working on this project. It's an invaluable help when developing new features (finding the reason a piece of code is here) or finding regressions (see if the commit could potentially affect the observed behavior) alike.

Our contributing guidelines include an article about good commit messages which is well worth the read.

[dbankov-vmware]

Updated title and description for both the PR and the commit.
I looked at https://github.com/dlang/dub/blob/master/CONTRIBUTING.md before filing the PR which referred to https://github.blog/2013-01-22-closing-issues-via-commit-messages/ which left me with the impression that the commit message should include only 'Fixes...'.

[Geod24]

Thanks!

I looked at https://github.com/dlang/dub/blob/master/CONTRIBUTING.md before filing the PR which referred to https://github.blog/2013-01-22-closing-issues-via-commit-messages/ which left me with the impression that the commit message should include only 'Fixes...'.

Thanks for the feedback, I'll look at updating the contributing guide to make requirements clearer.