posix: consume less entropy on tempname

The first getrandom is used only for __GT_NOCREATE, which is inherently
insecure and can use the entropy as a small improvement.  On the
second and later attempts it might help against DoS attacks.

It sync with gnulib commit 854fbb81d91f7a0f2b463e7ace2499dee2f380f2.

Checked on x86_64-linux-gnu.