[vulkan]: Implement VkSemaphore using Marl primitives.

Proper implementation of a VkSemaphore using an std::mutex
and a marl::ConditionVariable. This should make the implementation
compatible with both fibers and threads at the same time.

A future CL will add platform-specific implementations to cover
external semaphores as well. Which explains why the implementation
details are hidden from VkSemaphore.hpp.

Bug: b/140421726
Change-Id: I1db55493a41db0eb60ce9181fe864253db09f4f8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35933
Tested-by: David Turner <digit@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

src/Vulkan/BUILD.gn

diff --git a/src/Vulkan/BUILD.gn b/src/Vulkan/BUILD.gn
index 8c6141b..8e19658 100644
--- a/src/Vulkan/BUILD.gn
+++ b/src/Vulkan/BUILD.gn
@@ -113,6 +113,7 @@
     "VkQueue.cpp",
     "VkRenderPass.cpp",
     "VkSampler.cpp",
+    "VkSemaphore.cpp",
     "VkShaderModule.cpp",
     "Vulkan.rc",
     "libVulkan.cpp",