Fix arm[64] cflags

Limit arm cpu variants to armv7-a-neon only, and handle a generic
armv7-a-neon cpu variant. Add arm64 cortex-a53 cpu variant. Sanity check
the arch variant against supported versions.

Change-Id: I8ecdea4cd36d9c82e1f29b158efc131f8b1cbc4d
diff --git a/cc/util.go b/cc/util.go
index c083744..1d27f73 100644
--- a/cc/util.go
+++ b/cc/util.go
@@ -86,3 +86,22 @@
 		clang:       in.Clang,
 	}
 }
+
+func copyVariantFlags(m map[string][]string) map[string][]string {
+	ret := make(map[string][]string, len(m))
+	for k, v := range m {
+		l := make([]string, len(m[k]))
+		for i := range m[k] {
+			l[i] = v[i]
+		}
+		ret[k] = l
+	}
+	return ret
+}
+
+func variantOrDefault(variants map[string]string, choice string) string {
+	if ret, ok := variants[choice]; ok {
+		return ret
+	}
+	return variants[""]
+}