elf, nptl: Initialize static TLS directly in ld.so

The stack list is available in ld.so since commit
1daccf40 ("nptl: Move stack list
variables into _rtld_global"), so it's possible to walk the stack
list directly in ld.so and perform the initialization there.

This eliminates an unprotected function pointer from _rtld_global
and reduces the libpthread initialization code.