Fix crossgen2 gc layout handling for TypedReference (#555)

Fix crossgen2 gc layout handling for TypedReference (#555)

* Fix crossgen2 gc layout handling for TypedReference

The getClassGCLayout jit interface method was not handling TypedReference
explicitly like its runtime counterpart. That was causing the layout to
be incorrect for TypedReference and resulted in 100 failures (asserts)
in the coreclr pri1 tests build and one framework assembly build with
crossgen2.
The assert in JIT was in Linux specific code path, so we weren't hitting
it on Windows.
With this fix, the build and test run results match the Windows ones.

* Remove the System.Runtime.Serialization.Formatters from exclusions

The assembly is now compiled ok