gc: look at cumulative error count, not just per-function.

Not sure if this is what you'd really want.  Maybe with a higher limit than 10
or perhaps keep checking nerrors > 10 per yyerror, but check the cumulative
after each function?

R=rsc
CC=golang-dev
https://golang.org/cl/5376064

src/cmd/gc/subr.c

@@ -119,7 +119,7 @@ yyerrorl(int line, char *fmt, ...)
 
 	hcrash();
 	nerrors++;
-	if(nerrors >= 10 && !debug['e']) {
+	if(nsavederrors+nerrors >= 10 && !debug['e']) {
 		flusherrors();
 		print("%L: too many errors\n", line);
 		errorexit();
@@ -187,7 +187,7 @@ yyerror(char *fmt, ...)
 
 	hcrash();
 	nerrors++;
-	if(nerrors >= 10 && !debug['e']) {
+	if(nsavederrors+nerrors >= 10 && !debug['e']) {
 		flusherrors();
 		print("%L: too many errors\n", parserline());
 		errorexit();