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/commands/nativedisk.c

@@ -79,6 +79,7 @@ get_uuid (const char *name, char **uuid, int getnative)
     case GRUB_DISK_DEVICE_XEN:
       if (getnative)
 	break;
+      /* FALLTHROUGH */
 
       /* Virtual disks.  */
       /* GRUB dynamically generated files.  */