Derive _targetOS from RuntimeIdentifier

src/coreclr/nativeaot/BuildIntegration/Microsoft.DotNet.ILCompiler.SingleEntry.targets

@@ -2,23 +2,26 @@
 
   <PropertyGroup Condition="'$(RuntimeIdentifier)' != '' and '$(NETCoreSdkPortableRuntimeIdentifier)' != ''">
     <!-- Define the name of the runtime specific compiler package to import -->
-    <_hostOS>$(NETCoreSdkPortableRuntimeIdentifier.SubString(0, $(NETCoreSdkPortableRuntimeIdentifier.IndexOf('-'))))</_hostOS>
+    <_hostOS>$(NETCoreSdkPortableRuntimeIdentifier.SubString(0, $(NETCoreSdkPortableRuntimeIdentifier.LastIndexOf('-'))))</_hostOS>
 
-    <_useTPI Condition="'$(TargetPlatformIdentifier)' != '' and '$(TargetPlatformIdentifier)' != 'unix'">true</_useTPI>
-    <_targetOS Condition="'$(_useTPI)' == 'true'">$(TargetPlatformIdentifier.ToLowerInvariant)</_targetOS>
-    <_targetOS Condition="'$(_useTPI)' != 'true'">$(_hostOS)</_targetOS>
+    <_targetOS>$(RuntimeIdentifier.SubString(0, $(RuntimeIdentifier.LastIndexOf('-'))))</_targetOS>
+    <_indexOfPeriod>$(_targetOS.IndexOf('.'))</_indexOfPeriod>
+    <_targetOS Condition="'$(_indexOfPeriod)' &gt; -1">$(_targetOS.SubString(0, $(_indexOfPeriod)))</_targetOS>
     <_targetOS Condition="$(_targetOS.StartsWith('win'))">win</_targetOS>
 
     <!-- On non-Windows, determine _hostArchitecture from NETCoreSdkPortableRuntimeIdentifier -->
     <_hostArchitecture Condition="'$(OS)' != 'Windows_NT'">$(NETCoreSdkPortableRuntimeIdentifier.SubString($([MSBuild]::Add($(NETCoreSdkPortableRuntimeIdentifier.LastIndexOf('-')), 1))))</_hostArchitecture>
     <!-- On Windows, default to host that matches OS architecture on Windows. It allows compilation using msbuild.exe that is x86 by default -->
-    <_hostArchitecture Condition="'$(OS)' == 'Windows_NT'>$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture.ToString().ToLowerInvariant)</_hostArchitecture>
+    <_hostArchitecture Condition="'$(OS)' == 'Windows_NT'">$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture.ToString().ToLowerInvariant)</_hostArchitecture>
 
     <!-- Determine _targetArchitecture from RuntimeIdentifier -->
     <_targetArchitecture>$(RuntimeIdentifier.SubString($([MSBuild]::Add($(RuntimeIdentifier.LastIndexOf('-')), 1))))</_targetArchitecture>
 
     <_hostPackageName>runtime.$(_hostOS)-$(_hostArchitecture).Microsoft.DotNet.ILCompiler</_hostPackageName>
-    <_targetPackageName>runtime.$(_targetOS)-$(_targetArchitecture).Microsoft.DotNet.ILCompiler</_targetPackageName>
+    <_targetPackageName>runtime.$(_targetOSPkg)-$(_targetArchitecture).Microsoft.DotNet.ILCompiler</_targetPackageName>
+
+    <!-- From this point onwards, we will treat linux-musl as linux -->
+    <_targetOS Condition="$(_targetOS.StartsWith('linux'))">linux</_targetOS>
 
     <IlcCalledViaPackage>true</IlcCalledViaPackage>
   </PropertyGroup>

src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Publish.targets

@@ -67,7 +67,7 @@
     <Error Condition="'$(DisableUnsupportedError)' != 'true' and !('$(_hostArchitecture)' == 'x64' or '$(_hostArchitecture)' == 'arm64')"
       Text="Native compilation can run on x64 and arm64 hosts only." />
 
-    <Error Condition="'$(IlcHostPackagePath)' == '' and '$(RuntimePackagePath)' != '' and ('$(_hostArchitecture)' == 'x64' or '$(_hostArchitecture)' == 'arm64')"
+    <Error Condition="'$(_hostPackageName)' == '' and '$(RuntimePackagePath)' != '' and ('$(_hostArchitecture)' == 'x64' or '$(_hostArchitecture)' == 'arm64')"
       Text="Add a PackageReference for '$(_hostPackageName)' to allow cross-compilation for $(_targetArchitecture)" />
 
     <!-- NativeAOT runtime pack assemblies need to be defined to avoid the default CoreCLR implementations being set as compiler inputs -->