Enable hosted builds on arm64

With this commit, I can build core-sdk on RHEL 8 on arm64 directly,
without cross compilation.

Bump the sourcelink version to pick up the ability to parse git info
without depending on libgit2sharp. This allows sourcelink to work on
arm64. The version is the same as the one recently added to core-setup:
dotnet/core-setup#7696

Introduce a new 'HostArch' msbuild property that contains the host
architecture (arm64, x64, etc). This is the architecture of the
currently running machine, and may be different from the architecture we
are targetting in the case of cross compilation.

There's a gotcha with HostArch: under Visual Studio (an x86) process,
we generally want a x64 architecture.

Use HostArch to determine whether _crossDir and LibCLRJitRid need to be
special-cased for arm64 or or not.

Directory.Build.props

@@ -7,6 +7,12 @@
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <HostArch>$([System.Runtime.InteropServices.RuntimeInformation]::ProcessArchitecture.ToString().ToLowerInvariant())</HostArch>
+    <HostArch Condition="'$(BuildingInsideVisualStudio)' == 'true'">x64</HostArch>
+    <Architecture Condition="'$(Architecture)' == ''">$(HostArch)</Architecture>
+  </PropertyGroup>
+
   <PropertyGroup>
     <TreatWarningsAsErrors>True</TreatWarningsAsErrors>
     <DebugType>embedded</DebugType>

eng/Versions.props

@@ -87,7 +87,7 @@
     <VersionToolsVersion>$(BuildTasksFeedToolVersion)</VersionToolsVersion>
     <DotnetDebToolVersion>2.0.0</DotnetDebToolVersion>
     <MicrosoftNETTestSdkVersion>15.8.0</MicrosoftNETTestSdkVersion>
-    <MicrosoftSourceLinkVersion>1.0.0-beta2-18618-05</MicrosoftSourceLinkVersion>
+    <MicrosoftSourceLinkVersion>1.0.0-beta2-19367-01</MicrosoftSourceLinkVersion>
   </PropertyGroup>
   <PropertyGroup>
     <!-- pinned dependency. This package is not being produced outside of the 2.0 branch of corefx and should not change. -->

src/redist/targets/Crossgen.targets

@@ -5,14 +5,14 @@
 
     <PropertyGroup>
       <RuntimeNETCoreAppPackageName>runtime.$(SharedFrameworkRid).microsoft.netcore.app</RuntimeNETCoreAppPackageName>
-      <_crossDir Condition="'$(Architecture)' == 'arm64'">/x64_arm64</_crossDir>
+      <_crossDir Condition="'$(Architecture)' == 'arm64' and '$(HostArch)' == 'x64'">/x64_arm64</_crossDir>
       <_crossDir Condition="'$(Architecture)' == 'arm' And '$(OSName)' == 'win'">/x86_arm</_crossDir>
       <_crossDir Condition="'$(Architecture)' == 'arm' And '$(OSName)' == 'linux'">/x64_arm</_crossDir>
       <CrossgenPath>$(NuGetPackageRoot)/$(RuntimeNETCoreAppPackageName)/$(MicrosoftNETCoreAppPackageVersion)/tools$(_crossDir)/crossgen$(ExeExtension)</CrossgenPath>
-      <LibCLRJitRid Condition="!$(Architecture.StartsWith('arm'))">$(SharedFrameworkRid)</LibCLRJitRid>
-      <LibCLRJitRid Condition="'$(Architecture)' == 'arm64'">x64_arm64</LibCLRJitRid>
+      <LibCLRJitRid Condition="'$(Architecture)' == 'arm64' and '$(HostArch)' == 'x64'">x64_arm64</LibCLRJitRid>
       <LibCLRJitRid Condition="'$(Architecture)' == 'arm' And '$(OSName)' == 'win'">x86_arm</LibCLRJitRid>
       <LibCLRJitRid Condition="'$(Architecture)' == 'arm' And '$(OSName)' == 'linux'">x64_arm</LibCLRJitRid>
+      <LibCLRJitRid Condition="'$(LibCLRJitRid)' == ''">$(SharedFrameworkRid)</LibCLRJitRid>
       <LibCLRJitPath>$(NuGetPackageRoot)/$(RuntimeNETCoreAppPackageName)/$(MicrosoftNETCoreAppPackageVersion)/runtimes/$(LibCLRJitRid)/native/$(DynamicLibPrefix)clrjit$(DynamicLibExtension)</LibCLRJitPath>
       <SharedFrameworkNameVersionPath>$(RedistLayoutPath)shared/$(SharedFrameworkName)/$(MicrosoftNETCoreAppPackageVersion)</SharedFrameworkNameVersionPath>
       <DIASymReaderCrossgenFilter>*</DIASymReaderCrossgenFilter>