[vulkan] Fuchsia build support

This initial patch attempts to minimize changes to existing code
by adding a fuchsia compat layer that matches what is expected
from android/linux. Multi platform support can be improved in
follow up changes by introducing a more appropriate platform layer.

BUILD.gn is limited to target_os=fuchsia but can be improved to
support other platforms.

Change-Id: I7c69a4368d3560032ff341ee8a5a58c01b50ab5e