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)
3 jobs for server-1.20-branch in 3 minutes and 58 seconds
Status Job ID Name Coverage
  Docker Image
passed #5327690
debian-testing

00:00:20

 
  Build And Test
passed #5327691
autotools-build-and-test

00:03:37

passed #5327692
meson-build-and-test

00:01:09