Fix failed 'server' platform assets on Linux related to Shaders (#4275)

* Add missing 'server' platform identifier for the ShaderBuilder
* Use the current host platform as the fallback platform identifier (and not 'pc')

Signed-off-by: Steve Pham <spham@amazon.com>

Gems/Atom/Asset/Shader/Code/Source/Editor/ShaderBuilderUtility.cpp

@@ -37,6 +37,7 @@
 #include <AzslCompiler.h>
 
 #include "ShaderPlatformInterfaceRequest.h"
+#include "ShaderBuilder_Traits_Platform.h"
 #include "AtomShaderConfig.h"
 
 #include "SrgLayoutUtility.h"
@@ -456,8 +457,9 @@ namespace AZ
                 const uint32_t rhiUniqueIndex, const AZStd::string& platformIdentifier, const AZStd::string& shaderJsonPath,
                 const uint32_t supervariantIndex, RPI::ShaderAssetSubId shaderAssetSubId)
             {
-                // platform id from identifier
-                AzFramework::PlatformId platformId = AzFramework::PlatformId::PC;
+                // Define a fallback platform ID based on the current host platform
+                AzFramework::PlatformId platformId = AZ_TRAIT_ATOM_FALLBACK_ASSET_HOST_PLATFORM;
+
                 if (platformIdentifier == "pc")
                 {
                     platformId = AzFramework::PlatformId::PC;
@@ -478,6 +480,10 @@ namespace AZ
                 {
                     platformId = AzFramework::PlatformId::IOS;
                 }
+                else if (platformIdentifier == "server")
+                {
+                    platformId = AzFramework::PlatformId::SERVER;
+                }
 
                 uint32_t assetSubId = RPI::ShaderAsset::MakeProductAssetSubId(rhiUniqueIndex, supervariantIndex, aznumeric_cast<uint32_t>(shaderAssetSubId));
                 auto assetIdOutcome = RPI::AssetUtils::MakeAssetId(shaderJsonPath, assetSubId);

Gems/Atom/Asset/Shader/Code/Source/Platform/Android/ShaderBuilder_Traits_Android.h

@@ -8,3 +8,4 @@
 #pragma once
 
 #define AZ_TRAIT_ATOM_SHADERBUILDER_AZSLC UNUSED_TRAIT
+#define AZ_TRAIT_ATOM_FALLBACK_ASSET_HOST_PLATFORM UNUSED_TRAIT

Gems/Atom/Asset/Shader/Code/Source/Platform/Linux/ShaderBuilder_Traits_Linux.h

@@ -8,4 +8,4 @@
 #pragma once
 
 #define AZ_TRAIT_ATOM_SHADERBUILDER_AZSLC "azslc"
-
+#define AZ_TRAIT_ATOM_FALLBACK_ASSET_HOST_PLATFORM AzFramework::PlatformId::LINUX_ID

Gems/Atom/Asset/Shader/Code/Source/Platform/Mac/ShaderBuilder_Traits_Mac.h

@@ -8,3 +8,4 @@
 #pragma once
 
 #define AZ_TRAIT_ATOM_SHADERBUILDER_AZSLC "azslc"
+#define AZ_TRAIT_ATOM_FALLBACK_ASSET_HOST_PLATFORM AzFramework::PlatformId::MAC_ID

Gems/Atom/Asset/Shader/Code/Source/Platform/Windows/ShaderBuilder_Traits_Windows.h

@@ -8,3 +8,4 @@
 #pragma once
 
 #define AZ_TRAIT_ATOM_SHADERBUILDER_AZSLC "azslc.exe"
+#define AZ_TRAIT_ATOM_FALLBACK_ASSET_HOST_PLATFORM AzFramework::PlatformId::PC

Gems/Atom/Asset/Shader/Code/Source/Platform/iOS/ShaderBuilder_Traits_iOS.h

@@ -8,3 +8,4 @@
 #pragma once
 
 #define AZ_TRAIT_ATOM_SHADERBUILDER_AZSLC UNUSED_TRAIT
+#define AZ_TRAIT_ATOM_FALLBACK_ASSET_HOST_PLATFORM UNUSED_TRAIT