Support building for arm64 on arm64 (*nix) (#15354)

This commit allows ASP.NET Core to be built on arm64 machines directly,
without relying on cross-compilation.

There's a few changes in here:

1. Ask msbuild to look into the BuildArchitecture

   By default, our build systems assums the machine is x64. This
   modifies the build configuration to check the architecture of the
   currently running build machine, and set BuildArchitecture to that.

2. Fix crossgen in Microsoft.AspNetCore.App.Runtime

   We run crossgen for supported architectures (including x64 and
   arm64). For that, we need a jit that we can point crossgen to.
   Generally, we can rely on the build scripts to find the right
   `libclrjit.so`. However, arm64 has multiple `libclirjit.so`, for
   different use-cases. There's one for arm64 (for running on arm64) and
   there's another one for cross-compiling for arm64 on x64. We need to
   figure out and use the right one explicitly rather than assuming the
   right one gets picked up.

   See dotnet/core-setup#8468 for similar
   changes made in core-setup.

This also needs #14790 to fully
work on arm64.

Directory.Build.props

@@ -108,6 +108,7 @@
     <TargetOsName Condition=" '$(TargetOsName)' == '' AND $([MSBuild]::IsOSPlatform('Windows'))">win</TargetOsName>
     <TargetOsName Condition=" '$(TargetOsName)' == '' AND $([MSBuild]::IsOSPlatform('OSX'))">osx</TargetOsName>
     <TargetOsName Condition=" '$(TargetOsName)' == '' AND $([MSBuild]::IsOSPlatform('Linux'))">linux</TargetOsName>
+    <BuildArchitecture>$([System.Runtime.InteropServices.RuntimeInformation]::ProcessArchitecture.ToString().ToLowerInvariant())</BuildArchitecture>
     <TargetArchitecture Condition="'$(TargetArchitecture)' == ''">x64</TargetArchitecture>
     <TargetRuntimeIdentifier>$(TargetOsName)-$(TargetArchitecture)</TargetRuntimeIdentifier>
 

src/Framework/src/Microsoft.AspNetCore.App.Runtime.csproj

@@ -90,15 +90,17 @@ This package is an internal implementation of the .NET Core SDK and is not meant
     <PathSeparator Condition="'$(PathSeparator)' == ''">:</PathSeparator>
     <PathSeparator Condition=" '$(TargetOsName)' == 'win' ">%3B</PathSeparator>
 
+    <CrossCompileDirectory Condition=" '$(TargetRuntimeIdentifier)' == 'linux-arm' ">x64_arm</CrossCompileDirectory>
+    <CrossCompileDirectory Condition=" '$(TargetArchitecture)' == 'arm64' AND '$(BuildArchitecture)' != 'arm64' ">x64_arm64</CrossCompileDirectory>
+    <CrossCompileDirectory Condition=" '$(TargetRuntimeIdentifier)' == 'win-arm' ">x86_arm</CrossCompileDirectory>
+
     <!-- Crossgen executable name -->
     <CrossgenToolFileName>crossgen</CrossgenToolFileName>
     <CrossgenToolFileName Condition=" '$(TargetOsName)' == 'win' ">$(CrossgenToolFileName).exe</CrossgenToolFileName>
     <!-- Default crossgen executable relative path -->
     <CrossgenToolPackagePath>$(CrossgenToolFileName)</CrossgenToolPackagePath>
     <!-- Disambiguated RID-specific crossgen executable relative path -->
-    <CrossgenToolPackagePath Condition=" '$(TargetRuntimeIdentifier)' == 'linux-arm' ">x64_arm\$(CrossgenToolPackagePath)</CrossgenToolPackagePath>
-    <CrossgenToolPackagePath Condition=" '$(TargetRuntimeIdentifier)' == 'linux-arm64' OR '$(TargetRuntimeIdentifier)' == 'linux-musl-arm64' ">x64_arm64\$(CrossgenToolPackagePath)</CrossgenToolPackagePath>
-    <CrossgenToolPackagePath Condition=" '$(TargetRuntimeIdentifier)' == 'win-arm' ">x86_arm\$(CrossgenToolPackagePath)</CrossgenToolPackagePath>
+    <CrossgenToolPackagePath Condition=" '$(CrossCompileDirectory)' != '' ">$(CrossCompileDirectory)\$(CrossgenToolPackagePath)</CrossgenToolPackagePath>
 
     <RuntimePackageRoot>$([System.IO.Path]::Combine('$(NuGetPackageRoot)', 'microsoft.netcore.app.runtime.$(RuntimeIdentifier)', '$(MicrosoftNETCoreAppRuntimeVersion)'))</RuntimePackageRoot>
     <RuntimePackageRoot>$([MSBuild]::EnsureTrailingSlash('$(RuntimePackageRoot)'))</RuntimePackageRoot>
@@ -293,7 +295,9 @@ This package is an internal implementation of the .NET Core SDK and is not meant
   -->
   <PropertyGroup>
     <CrossgenToolDir>$(IntermediateOutputPath)crossgen\</CrossgenToolDir>
-    <CoreCLRJitPath>$(CrossgenToolDir)$(LibPrefix)clrjit$(LibExtension)</CoreCLRJitPath>
+    <!-- Pick the right coreclr jit based on whether we are cross-compiling or not -->
+    <CoreCLRJitPath Condition="'$(CrossCompileDirectory)' == ''">$(RuntimePackageRoot)runtimes\$(RuntimeIdentifier)\native\$(LibPrefix)clrjit$(LibExtension)</CoreCLRJitPath>
+    <CoreCLRJitPath Condition="'$(CrossCompileDirectory)' != ''">$(RuntimepackageRoot)runtimes\$(CrossCompileDirectory)\native\$(LibPrefix)clrjit$(LibExtension)</CoreCLRJitPath>
   </PropertyGroup>
 
   <ItemGroup>