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

util/getroot.c

@@ -99,7 +99,7 @@ grub_util_pull_device (const char *os_dev)
     {
     case GRUB_DEV_ABSTRACTION_LVM:
       grub_util_pull_lvm_by_command (os_dev);
-      /* Fallthrough in case that lvm-tools are unavailable.  */
+      /* Fallthrough - in case that lvm-tools are unavailable.  */
     case GRUB_DEV_ABSTRACTION_LUKS:
       grub_util_pull_devmapper (os_dev);
       return;

util/grub-install.c

@@ -1851,6 +1851,7 @@ main (int argc, char *argv[])
 	  free (mach_kernel);
 	  break;
 	}
+      /* FALLTHROUGH */
     case GRUB_INSTALL_PLATFORM_ARM_EFI:
     case GRUB_INSTALL_PLATFORM_ARM64_EFI:
     case GRUB_INSTALL_PLATFORM_IA64_EFI:

util/grub-mkimagexx.c

@@ -907,6 +907,7 @@ SUFFIX (relocate_addresses) (Elf_Ehdr *e, Elf_Shdr *sections,
 									    + sym->st_value
 									    - image_target->vaddr_offset));
 		  }
+		/* FALLTHROUGH */
 		case R_IA64_LTOFF_FPTR22:
 		  *gpptr = grub_host_to_target64 (addend + sym_addr);
 		  grub_ia64_add_value_to_slot_21 ((grub_addr_t) target,

util/grub-mount.c

@@ -530,6 +530,7 @@ argp_parser (int key, char *arg, struct argp_state *state)
       if (arg[0] != '-')
 	break;
 
+    /* FALLTHROUGH */
     default:
       if (!arg)
 	return 0;