Reduce parallelism for composite crossgen2 for jit64_3 test (#128269)

win-x86 legs were OOM-ing because methods from HugeArray, HugeField,
etc, were being compiled in parallel and using excessive amounts of
native memory. Reducing parallelism to 1 for composite mode fixes the
issue without making build times excessive everywhere. We could consider
limiting the scope further to only 32-bit platforms as well.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jtschuster <36744439+jtschuster@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

src/tests/Common/CLRTest.CrossGen.targets

@@ -32,6 +32,10 @@ WARNING:   When setting properties based on their current state (for example:
     Tests can opt out of ahead-of-time (AOT) compilation from crossgen / crossgen2 by setting this property in their project:
 
       <CrossGenTest>false</CrossGenTest>
+
+    Tests can pass extra crossgen2 arguments only in composite mode by setting this property in their project:
+
+      <CompositeCrossGen2TestExtraArguments>...</CompositeCrossGen2TestExtraArguments>
   -->
   <Target Name="GetCrossgenBashScript">
 
@@ -67,6 +71,7 @@ if [ ! -z ${RunCrossGen2+x} ]%3B then
         cp "$CORE_ROOT"/lib*.so "$CORE_ROOT"/lib*.dylib $(scriptPath)
 
         ExtraCrossGen2Args+=" --composite"
+        ExtraCrossGen2Args+=" $(CompositeCrossGen2TestExtraArguments)"
       fi
 
       ExtraCrossGen2Args+=" $(CrossGen2TestExtraArguments)"
@@ -261,6 +266,7 @@ if defined RunCrossGen2 (
 
     if defined CompositeBuildMode (
         set ExtraCrossGen2Args=!ExtraCrossGen2Args! --composite
+        set ExtraCrossGen2Args=!ExtraCrossGen2Args! $(CompositeCrossGen2TestExtraArguments)
         if "$(CrossGen2OutputFormat)"=="wasm" (
             set __OutputFile=!scriptPath!\composite-r2r.wasm
         ) else (

src/tests/JIT/jit64/jit64_3.csproj

@@ -1,4 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Huge methods need large native JIT allocations, so limit concurrent Crossgen2 compilations. -->
+    <CompositeCrossGen2TestExtraArguments>$(CompositeCrossGen2TestExtraArguments) --parallelism:1</CompositeCrossGen2TestExtraArguments>
+  </PropertyGroup>
+
   <ItemGroup>
     <MergedWrapperProjectReference Include="opt/**/*.??proj" />
   </ItemGroup>