patch 8.0.1215: newer gcc warns for implicit fallthrough

Problem:    Newer gcc warns for implicit fallthrough.
Solution:   Consistently use a FALLTHROUGH comment. (Christian Brabandt)

src/regexp.c

@@ -1997,7 +1997,7 @@ regatom(int *flagp)
 	    goto collection;
 
 	/* "\_x" is character class plus newline */
-	/*FALLTHROUGH*/
+	/* FALLTHROUGH */
 
 	/*
 	 * Character classes.
@@ -5847,7 +5847,7 @@ regrepeat(
       case IDENT:
       case IDENT + ADD_NL:
 	testval = TRUE;
-	/*FALLTHROUGH*/
+	/* FALLTHROUGH */
       case SIDENT:
       case SIDENT + ADD_NL:
 	while (count < maxcount)
@@ -5877,7 +5877,7 @@ regrepeat(
       case KWORD:
       case KWORD + ADD_NL:
 	testval = TRUE;
-	/*FALLTHROUGH*/
+	/* FALLTHROUGH */
       case SKWORD:
       case SKWORD + ADD_NL:
 	while (count < maxcount)
@@ -5908,7 +5908,7 @@ regrepeat(
       case FNAME:
       case FNAME + ADD_NL:
 	testval = TRUE;
-	/*FALLTHROUGH*/
+	/* FALLTHROUGH */
       case SFNAME:
       case SFNAME + ADD_NL:
 	while (count < maxcount)
@@ -5938,7 +5938,7 @@ regrepeat(
       case PRINT:
       case PRINT + ADD_NL:
 	testval = TRUE;
-	/*FALLTHROUGH*/
+	/* FALLTHROUGH */
       case SPRINT:
       case SPRINT + ADD_NL:
 	while (count < maxcount)
@@ -6131,7 +6131,7 @@ do_class:
       case ANYOF:
       case ANYOF + ADD_NL:
 	testval = TRUE;
-	/*FALLTHROUGH*/
+	/* FALLTHROUGH */
 
       case ANYBUT:
       case ANYBUT + ADD_NL: