tzcode: update to 2015b

Changes affecting code

Fix integer overflow bug in reference 'mktime' implementation.
(Problem reported by Jörg Richter.)

Allow -Dtime_tz=time_t compilations, and allow -Dtime_tz=... libraries
to be used in the same executable as standard-library time_t functions.
(Problems reported by Bradley White.)

Changes affecting commentary

Cite the recent Mexican decree changing Quintana Roo's time zone.
(Thanks to Carlos Raúl Perasso.)

Likewise for the recent Chilean decree.  (Thanks to Eduardo Romero Urra.)

Update info about Mars time.

(From OE-Core rev: fbd98e67)

Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>