[corlib] Rewrite RuntimeType.GetGenericParameterConstraints (#3260)

[corlib] Rewrite RuntimeType.GetGenericParameterConstraints (#3260)

* [corlib] Rewrite RuntimeType.GetGenericParameterConstraints

using managed code for managed object manipulation by surfacing the
native MonoGenericParamInfo struct to managed code as Mono.RuntimeStructs.GenericParamInfo

* [corlib] RuntimeType::GetGenericParameterAttributes can be managed code now.

* [corlib] Fix x86 breakage

RuntimeTypeHandle.GetGenericParameterInfo returns a pointer
Not a struct that's the same size as a pointer.  On the x86 Linux
calling convention they behave differently.