Merged PR 21869: Correct --architecture in fpm commands

# {PR title}

Correct `--architecture` in `fpm` commands

## Description

Update `fpm` commands to use a supported `--architecture` value. As-is, the x64 .rpm files we produce are incompatible w/ installation on an x64 machine. Problem found during servicing version flow.

## Customer Impact

Unable to build dotnet-installer-ci-official w/ current .rpm files from dotnet-aspnetcore repo.

## Regression?

- [x] Yes
- [ ] No

This is a build regression since 6.0.3. It will also impact our 'main' branch.

## Risk

- [ ] High
- [ ] Medium
- [x] Low

Have verified the chosen values for the command line using available online documentation. Since we know the current .rpm files are busted, things can only get better 😃

## Verification

- [x] Manual (required)
- [ ] Automated

I am locally building this branch and will use `rpmlint` in a `docker` container to verify the goodness of the produced package.

## Packaging changes reviewed?

- [ ] Yes
- [ ] No
- [ ] N/A

Not sure how to answer this because the change is to an internal setting of the RPM installer. If others agree it's the right change, it's been reviewed 😃

----

## When servicing release/2.1

- [ ] Make necessary changes in eng/PatchConfig.props

src/Installers/Rpm/Directory.Build.targets

@@ -50,6 +50,14 @@
       <ChangeLogProps>$(ChangeLogProps);PACKAGE_VERSION=$(PackageVersion)</ChangeLogProps>
       <ChangeLogProps>$(ChangeLogProps);PACKAGE_REVISION=$(PackageRevision)</ChangeLogProps>
 
+      <!--
+        RpmArch is used to match our naming conventions but
+        https://fedoraproject.org/wiki/Architectures#Primary_Architectures lists the canonical names `fpm` and
+        `rpmbuild` expect on the command line. Aliases like ARMv8, arm-64 and amd64 should also work.
+      -->
+      <CommandLineArch Condition=" '$(TargetArchitecture)' == 'x64' ">x86_64</CommandLineArch>
+      <CommandLineArch Condition=" '$(TargetArchitecture)' == 'arm64' ">aarch64</CommandLineArch>
+
       <GeneratedChangeLog>$(IntermediateOutputPath)changelog</GeneratedChangeLog>
     </PropertyGroup>
 
@@ -64,7 +72,7 @@
       <FpmArgs Include="--package=$(TargetPath)" />
       <FpmArgs Include="--version=$(PackageVersion)" />
       <FpmArgs Include="--iteration=$(PackageRevision)" />
-      <FpmArgs Include="--architecture=$(RpmArch)" />
+      <FpmArgs Include="--architecture=$(CommandLineArch)" />
       <FpmArgs Include="--depends=&quot;%(RpmDependency.Identity) &gt;= %(RpmDependency.Version)&quot;" Condition=" '%(RpmDependency.Identity)' != '' "  />
       <FpmArgs Include="--rpm-changelog=&quot;$(GeneratedChangeLog)&quot;" />
       <FpmArgs Include="--rpm-summary=&quot;$(PackageSummary)&quot;" />