Reporting some issues on the platforms matching that appear on arm platform.

• The unit tests for platforms pkg do not pass if run on arm. They seem to be related to the issues described below.

• Implicit arm variant should be handled during Match() in the platform struct the same way as it is in the string form (and as it is defined in the docs). Currently containerd can never match an existing arm image that doesn't have the variant set. Even if the current system platform variant is v7.

• It is problematic that a behavior of a function like Parse() that is supposed to convert a string to a struct completely depends on the arch that is calling the function. This makes the Parse() function useless if the caller isn't running on the same node that is being checked (could possibly even apply to the current usage in containerd client library). I'd suggest letting parse return partial results and having separate function that can combine that with the result of Default(), and maybe combine them for current behavior in a function like ParseWithCurrentDefault().

@dmcgowan