os: Fix instruction pointer written in xorg_backtrace

The address retrieved in "pip.start_ip" is not necessarily the same
address as unw_get_proc_name finds as nearest symbol and returns in "off".
Therefore using "pip.start_ip + off" is not reliable, at least
visible in the binaries from the Debian repository.

Bug-Debian: https://bugs.debian.org/971088

Signed-off-by: Bernhard Übelacker <bernhardu@mailbox.org>
(cherry picked from commit c15dd0ba)
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