libvulkan: support vkGIPA self-query

vkGetInstanceProcAddr(nullptr, ...) is valid and is effectively a
globally dispatched function, so it's consistent to support querying
vkGetInstanceProcAddr from itself without an instance.

This is required in Vulkan 1.2 and is supported for all Vulkan versions
by the loader on other platforms. To maximize app compatibility and
minimize surprises, this makes it work on Android also, even for pre-1.2
versions where the result is undefined by the spec.

Bug: 157173922
Test: dEQP-VK.api.version_check.entry_points with
      https://gerrit.khronos.org/#/c/5490/ applied and modified to
      check even on pre-1.2 implementations.
Change-Id: I820dd1239df54a415b7ff5db47cf2c2b349f6155