Improve FreeBSD Support

[bashbaug]

see: KhronosGroup/OpenCL-SDK#55

A minimum we should modify the OpenCL ICD loader so it checks for FreeBSD or Unix generically vs. just Linux, to fix build issues.

Separately we can investigate renaming some of the source files so they apply more generically to Unix vs. Linux.

[juliohm]

Regarding the FreeBSD environment to test, the build script I provided in https://github.com/juliohm/cross-platform-opencl should be enough.

You can type

$ julia --project build_tarballs.jl x86_64-unknown-freebsd

to emulate a FreeBSD.

[bashbaug]

Thanks! I'll test your build script next. I also setup a FreeBSD VM so I can at least build and run unit tests.

[juliohm]

I confirm that #185 fixed the build on FreeBSD 👍🏽 Now we are totaling 14 platforms, only missing Windows.

I've updated the build script as well in case you want to give it a try or add it somewhere in CI to release the binaries for these platforms to end users.