Revert "libthr: Use kern.stacktop for thread stack calculation."

The current ASLR stack gap feature will be removed, and with that the
need for this change, and the kern.stactop sysctl, is gone. Moreover,
the approach taken in this revision does not provide compatibility for
old copies of libthr.so, and the revision should have also updated
__libc_map_stacks_exec().

This reverts commit 78df56ccfcb40013a3e6904bd6d39836220c3550.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f75b1ff6e5698ac092aa33d66ea73a56f381ed99)