[libc] implement tmpnam

Needed for libc++. Defined in the C standard on page 339: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=339

Start with the C standard's tmpnam specification on page 339 and determine how the libc++ requirement relates to the libc implementation. Review linked pull request #204901 for the work already underway; done means tmpnam is implemented according to the cited standard and satisfies the relevant libc++ use.

Written by the indexing model from the issue text.