cmd/gc: catch notes on Plan 9

LGTM=rsc
R=rsc, jas, gobot
CC=ality, golang-codereviews
https://golang.org/cl/51650051

src/cmd/gc/lex.c

@@ -164,6 +164,23 @@ fault(int s)
 	fatal("fault");
 }
 
+void
+catcher(void *v, char *s)
+{
+	USED(v);
+
+	if(strncmp(s, "sys: fp: invalid operation", 26) == 0) {
+		noted(NCONT);
+		return;
+	}
+	if(strncmp(s, "sys: trap: fault read", 21) == 0) {
+		if(nsavederrors + nerrors > 0)
+			errorexit();
+		fatal("fault");
+	}
+	noted(NDFLT);
+}
+
 void
 doversion(void)
 {
@@ -188,6 +205,10 @@ main(int argc, char *argv[])
 	signal(SIGSEGV, fault);
 #endif
 
+#ifdef	PLAN9
+	notify(catcher);
+#endif
+
 	ctxt = linknew(thelinkarch);
 	ctxt->diag = yyerror;
 	ctxt->bso = &bstdout;