libc: implement rtld_get_stack_prot() for real

[kib]

Summary

which makes stack prot correct for not main threads created by binaries with statically linked libthr.

Cache result, but do not engage into full double-checked locking, since calculation of the return value is idempotent.

PR: 252549

[emaste]

Looks good to me.

I wondered if we want to keep the calculated stack prot to avoid iterating over the phdrs for each created thread but the value seems limited, simplicity is probably better.

[kib]

Cache return value.