Stop relying on compiler to implicitly add pthread/math libraries

[marler8997]

This is required to be able to remove the hardcoded libraries that druntime/phobos depend on when the option -defaultlib= is given.

Was able to run the unittests with this change and dlang/dmd#9831

This PR is a small piece in working towards a "pay for what you use" model and removing unnecessary interdependency between the compiler and the standard library.

[dlang-bot]

Thanks for your pull request and interest in making D better, @marler8997! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + druntime#2619"