add `NATIVEAOT` constant when publish aot

[WeihanLi]

Is your feature request related to a problem? Please describe.

When publish with AOT, add a constant NATIVEAOT so that we could do aot-specific operation likes

#if NATIVEAOT
// .. aot-only
#endif

Describe the solution you'd like

When PublishAot is true, define additional constant

<PropertyGroup>
   <DefineConstants Condition="'$(PublishAot)'=='true'">$(DefineConstants);NATIVEAOT</DefineConstants> 
</PropertyGroup>

Additional context

see comment:
dotnet/runtime#109005 (reply in thread)