i386, x86_64, ppc: fix switch fallthrough cases with GCC7

In util/getroot and efidisk slightly modify exitsing comment to mostly
retain it but still make GCC7 compliant with respect to fall through
annotation.

In grub-core/lib/xzembed/xz_dec_lzma2.c it adds same comments as
upstream.

In grub-core/tests/setjmp_tets.c declare functions as "noreturn" to
suppress GCC7 warning.

In grub-core/gnulib/regexec.c use new __attribute__, because existing
annotation is not recognized by GCC7 parser (which requires that comment
immediately precedes case statement).

Otherwise add FALLTHROUGH comment.

Closes: 50598

grub-core/disk/efi/efidisk.c

@@ -224,7 +224,7 @@ name_devices (struct grub_efidisk_data *devices)
 	    {
 	    case GRUB_EFI_HARD_DRIVE_DEVICE_PATH_SUBTYPE:
 	      is_hard_drive = 1;
-	      /* Fall through by intention.  */
+	      /* Intentionally fall through.  */
 	    case GRUB_EFI_CDROM_DEVICE_PATH_SUBTYPE:
 	      {
 		struct grub_efidisk_data *parent, *parent2;