Fix arm platform matching

The normalization was being inconsistently applied causing a
failure to match some platforms in manifest lists.
Fix the matcher and normalization to be more consistent and
add changes to parser to prevent the defaulted variants from being
set in the platform structure.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

platforms/platforms.go

@@ -135,7 +135,7 @@ type Matcher interface {
 // Applications should opt to use `Match` over directly parsing specifiers.
 func NewMatcher(platform specs.Platform) Matcher {
 	return &matcher{
-		Platform: platform,
+		Platform: Normalize(platform),
 	}
 }
 
@@ -197,6 +197,9 @@ func Parse(specifier string) (specs.Platform, error) {
 		}
 
 		p.Architecture, p.Variant = normalizeArch(parts[0], "")
+		if defaultVariant(p.Architecture, p.Variant) {
+			p.Variant = ""
+		}
 		if isKnownArch(p.Architecture) {
 			p.OS = runtime.GOOS
 			return p, nil
@@ -208,6 +211,9 @@ func Parse(specifier string) (specs.Platform, error) {
 		// about whether or not we know of the platform.
 		p.OS = normalizeOS(parts[0])
 		p.Architecture, p.Variant = normalizeArch(parts[1], "")
+		if defaultVariant(p.Architecture, p.Variant) {
+			p.Variant = ""
+		}
 
 		return p, nil
 	case 3: