Exit gracefully when running ildasm -nat against unsupported ni file. (dotnet/coreclr...

Exit gracefully when running ildasm -nat against unsupported ni file. (dotnet/coreclr#10821)

ildasm -nat only works for fragile native images (ie CoreLib only by default).
Running ildasm -nat against unsupported native images would cause a segmentation fault. This PR checks pNativeHeader->Signature == CORCOMPILE_SIGNATURE and end the program with a meaningful error message.

fix dotnet/coreclr#10247

Commit migrated from https://github.com/dotnet/coreclr/commit/20248521af31e2307fcd728f1ebbf7877bb5ee59