layers: Add support for GNU/Hurd

[pinotree]

Add support for GNU/Hurd by following what done/used by other Unix platforms:

• use dlopen to load libraries
• add the PID to temporary files
• use the Vulkan XCB and XLIB platforms

It almost builds fine; the remaining bit to sort is layers/external/vma/vk_mem_alloc.h, which is a copy of a 3rd party sources: once that will be fixed, the next sync will hopefully solve the build fully.

The majority of the tests does not run, as there is no Vulkan device available yet (neither hardware nor software).

In case of doubts: yes, __GNU__ the official define for GNU/Hurd, see https://github.com/cpredef/predef/blob/master/OperatingSystems.md#gnu-aka-gnuhurd.

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CI Vulkan-ValidationLayers build queued with queue ID 204276.

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CI Vulkan-ValidationLayers build # 16883 running.

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CI Vulkan-ValidationLayers build # 16885 running.

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CI Vulkan-ValidationLayers build # 16885 passed.