Support long paths in CoreCLR runtime on Windows

Support long paths in CoreCLR runtime on Windows

The CoreCLR runtime is updated to support long file paths on Windows.
Pending updates to mscorlib.dll, the following scenarios are supported:
* Run managed apps from a long path.
* Load CoreCLR runtime and framework from a long path.
* Load user assemblies from a long path.
* Run CrossGen from a long path, with its input/output from a long path.
* Generate debug log file at a long path.

The following scenarios are not yet supported, and will be fixed in
future commits:
* Mscorlib.dll and framework assemblies are not yet long path compatible.
  Note that until mscorlib.dll is fixed, most of the runtime changes can't
  actually be used.
* Support on non-Windows platforms.
* Support for debugging and error reporting.
* Tools such as ilasm or ildasm.

Commit migrated from https://github.com/dotnet/coreclr/commit/488e6a16685241ed1aed3b3d386b78ed32e428f0