Output details for the faulting thread first in a core dump.

Patch from Matthias Schwarzott via BZ#315199.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14181 a5019735-40e9-0310-863c-91ae7b9d1cf9
Document fix of  BZ#315199.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14182 a5019735-40e9-0310-863c-91ae7b9d1cf9