Don't expose VK_EXT_external_memory_host on macOS

macOS has strong assumptions about GPU availability, and thus doesn't
have a reliable API for sharing memory between the GPU and SwiftShader
which can comply with SwiftShader's alignment and padding needs.
Specifically, we must not try obtaining a pointer from an IOSurface and
import it into SwiftShader.

Bug: angleproject:7538
Bug: b/178039602
Change-Id: I3ecc4dff31a9e025f50c4afcf7d82fa4384b2cd1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/67168
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>

src/Vulkan/libVulkan.cpp

diff --git a/src/Vulkan/libVulkan.cpp b/src/Vulkan/libVulkan.cpp
index 13c5ffb..77d33fa 100644
--- a/src/Vulkan/libVulkan.cpp
+++ b/src/Vulkan/libVulkan.cpp
@@ -368,9 +368,9 @@
 #if SWIFTSHADER_EXTERNAL_MEMORY_OPAQUE_FD
 	{ { VK_KHR_EXTERNAL_MEMORY_FD_EXTENSION_NAME, VK_KHR_EXTERNAL_MEMORY_FD_SPEC_VERSION } },
 #endif
-
+#if !defined(__APPLE__)
 	{ { VK_EXT_EXTERNAL_MEMORY_HOST_EXTENSION_NAME, VK_EXT_EXTERNAL_MEMORY_HOST_SPEC_VERSION } },
-
+#endif
 #if VK_USE_PLATFORM_FUCHSIA
 	{ { VK_FUCHSIA_EXTERNAL_SEMAPHORE_EXTENSION_NAME, VK_FUCHSIA_EXTERNAL_SEMAPHORE_SPEC_VERSION } },
 	{ { VK_FUCHSIA_EXTERNAL_MEMORY_EXTENSION_NAME, VK_FUCHSIA_EXTERNAL_MEMORY_SPEC_VERSION } },