Fix remaining cases of gcc 7 fallthrough warning.

They are all intended, so just add the relevant comment.

grub-core/kern/ia64/dl.c

@@ -119,6 +119,7 @@ grub_arch_dl_relocate_symbols (grub_dl_t mod, void *ehdr,
 	case R_IA64_LTOFF22:
 	  if (ELF_ST_TYPE (sym->st_info) == STT_FUNC)
 	    value = *(grub_uint64_t *) sym->st_value + rel->r_addend;
+	  /* Fallthrough.  */
 	case R_IA64_LTOFF_FPTR22:
 	  {
 	    grub_uint64_t *gpptr = mod->gotptr;

grub-core/kern/mips/dl.c

@@ -236,6 +236,7 @@ grub_arch_dl_relocate_symbols (grub_dl_t mod, void *ehdr,
 	      sym_value &= 0xffff0000;
 	      *(grub_uint16_t *) addr = 0;
 	    }
+	  /* Fallthrough.  */
 	case R_MIPS_CALL16:
 	  {
 	    grub_uint32_t *gpptr = mod->gotptr;

grub-core/kern/sparc64/dl.c

@@ -159,6 +159,7 @@ grub_arch_dl_relocate_symbols (grub_dl_t mod, void *ehdr,
 	  if (value >> 32)
 	    return grub_error (GRUB_ERR_BAD_MODULE,
 			       "address out of 32 bits range");
+	  /* Fallthrough.  */
 	case R_SPARC_LM22:
 	  *addr = (*addr & 0xFFC00000) | ((value >> 10) & 0x3FFFFF);
 	  break;

grub-core/loader/i386/coreboot/chainloader.c

@@ -384,6 +384,7 @@ load_chewed (grub_file_t file, const char *filename)
 	  segment.len = 0;
 	  segment.offset = 0;
 	  segment.len = 0;
+	  /* Fallthrough.  */
 	case PAYLOAD_SEGMENT_CODE:
 	case PAYLOAD_SEGMENT_DATA:
 	  {