Don't try to set RPATH on Windows

[MichalStrehovsky]

Should avoid

LINK : warning LNK4044: unrecognized option '/Wl,-rpath,'$ORIGIN/..''; ignored [d:\git\runtime2\src\tests\nativeaot\SmokeTests\UnitTests\UnitTests.csproj]

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Pull Request Overview

Avoid setting RPATH on Windows to suppress LNK4044 warnings during native AOT test builds.

• Tighten conditions so macOS and Apple mobile targets get their specific rpath flag.
• Apply $ORIGIN rpath only when TargetOS is neither Windows nor Apple platforms.
• Rename the Windows identifier literal from 'win' to 'windows'.

Comments suppressed due to low confidence (2)

src/tests/Directory.Build.targets:578

• Consider adding a CI test or build verification to assert that no -rpath flags are emitted when TargetOS is Windows to prevent regression of the LNK4044 warning.

<LinkerArg Include="-Wl,-rpath,'$ORIGIN/..'" Condition="'$(SetIlcRPath)' != 'false' and '$(TargetOS)' != 'windows' and '$(TargetOS)' != 'osx' and '$(TargetsAppleMobile)' != 'true'" />

src/tests/Directory.Build.targets:578

• The condition uses 'windows' but the actual TargetOS value may be 'Windows' (capitalized) or different; this mismatch could still apply the rpath flag on Windows. Ensure the literal and casing match the real TargetOS values.

<LinkerArg Include="-Wl,-rpath,'$ORIGIN/..'" Condition="'$(SetIlcRPath)' != 'false' and '$(TargetOS)' != 'windows' and '$(TargetOS)' != 'osx' and '$(TargetsAppleMobile)' != 'true'" />