Ensure Environment.ExitCode works correctly after app domain unloaded. (dotnet/corecl...

Ensure Environment.ExitCode works correctly after app domain unloaded. (dotnet/coreclr#10842)

* Ensure Environment.ExitCode works correctly after app domain unloaded.

This PR addresses 2 problems of Environment.ExitCode:

1. Can't get correct exit code of main function.
2. Can't set %errorlevel%.

Details can be found on dotnet/coreclr#6206

Fix dotnet/coreclr#6206

Commit migrated from https://github.com/dotnet/coreclr/commit/5e138a6d69215546e9bf4b38ec9bdd1c436fca5b