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/efiemu/mm.c

@@ -417,6 +417,7 @@ fill_hook (grub_uint64_t addr, grub_uint64_t size, grub_memory_type_t type,
       default:
 	grub_dprintf ("efiemu",
 		      "Unknown memory type %d. Assuming unusable\n", type);
+	/* FALLTHROUGH */
       case GRUB_MEMORY_RESERVED:
 	return grub_efiemu_add_to_mmap (addr, size,
 					GRUB_EFI_UNUSABLE_MEMORY);