slotinfo in struct dtv_slotinfo_list should be flexible array [BZ #25097]

GCC 10 will warn about subscribing inner length zero arrays.  Use a GCC
extension in csu/libc-tls.c to allocate space for the static_slotinfo
variable.  Adjust nptl_db so that the type description machinery does
not attempt to determine the size of the flexible array member slotinfo.

Change-Id: I51be146a7857186a4ede0bb40b332509487bdde8