Broken static builds on Windows

As previously discussed with some of the vendors, static builds on Windows haven't been working for some time. There's a lengthy disclaimer on why one wouldn't want to do this, but DLL handling is tedious enough that in controlled/development environments one may get away with it.

Some of the changes around ICD loading on Windows (summoning @jenatali) neatly reorganized things to headers/sources, but without the overhead of creating export headers and decorating functions when building a static library, one symbol ends up missing and the loader will not link. (Symbol export happens on a case-by-case basis based on usage. With multiple TUs, the compiler may not see the use of a symbol rendering it absent from the object file of the TU.)

Once the CI/CD changes make it into the repo, #226, I'd propose adding [this](https://github.com/StreamHPC/OpenCL-ICD-Loader/commit/c0aa73191b38d761574835d641291eb699f152c6) commit on top as a hotfix. It turns the Windows-specific parts of the loader into header-only, bringing function definitions closer to the location of use. This allows omission of generating an export header.

Start by reviewing the CI/CD changes in #226 and the Windows-specific loader headers and sources described in the issue. Compare the proposed commit c0aa73191b38d761574835d641291eb699f152c6, then verify that a Windows static build links successfully without a missing loader symbol.

Written by the indexing model from the issue text.