Prefer armv8.2-a to armv8.2a for GNU as compatibility

There are still out-of-tree users of `-fno-integrated-as`, which will
have issues without the hyphen.

Bug: http://b/158191033
Test: ./build.py
Change-Id: I44b68ab5b65a68df3993f38b99662178915e9be2
diff --git a/cc/config/arm64_device.go b/cc/config/arm64_device.go
index 9383463..62d8cc8 100644
--- a/cc/config/arm64_device.go
+++ b/cc/config/arm64_device.go
@@ -32,7 +32,7 @@
 			"-march=armv8-a",
 		},
 		"armv8-2a": []string{
-			"-march=armv8.2a",
+			"-march=armv8.2-a",
 		},
 	}