libthr: Remove the DTV_OFFSET macro.

This macro is confusing as it is not related to the similarly named
TLS_DTV_OFFSET. Instead, replace its one use with the desired
expression which is the same on all platforms.

Reviewed by: kib, emaste, jrtc27
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33345