Fix vulkan.pc Libs.private for static builds

Fix vulkan.pc Libs.private for static builds

The Libs.private field shouldn't be present for shared library builds,
so now the field wont be present in that case. For static library builds
it should be present, but the fields should be deduplicated as well.

Additionally, this adds a github actions run for testing the static
build of the loader on MacOS. This requires making the test framework
capable of using a statically built loader, which mainly took making
VulkanFunctions capable of assigning the statically defined functions
instead of loading them with dlsym.

The static build option required changes to the unix_shim.cpp to not
cause infinite loops.