arg(): allow redefining {BUILD,TARGET}{OS,ARCH,VARIANT}

Comments based on testing suggest that it's an incompatible mistake to
flag attempts to redefine TARGETOS, TARGETARCH, and TARGETVARIANT, but
it's okay to not allow TARGETPLATFORM to be redefined.  Do the same for
BUILD...

Signed-off-by: Nalin Dahyabhai <[email protected]>

dispatchers.go

@@ -755,7 +755,7 @@ func arg(b *Builder, args []string, attributes map[string]bool, flagArgs []strin
 		// If the stage introduces one of the predefined args, add the
 		// predefined value to the list of values known in this stage
 		if value, defined := builtinArgDefaults[name]; defined {
-			if haveDefault {
+			if haveDefault && (name == "TARGETPLATFORM" || name == "BUILDPLATFORM") {
 				return fmt.Errorf("attempted to redefine %q: %w", name, errdefs.ErrInvalidArgument)
 			}
 			if b.BuiltinArgDefaults == nil {