minidump-2-core: Output all memory regions to core file

minidump-2-core: Output all memory regions to core file

Most memory regions are already output to the core file due to the rest of
the minidump, but the region where the actual crashing PC is not.  The
backtrace command in gdb tries to disassemble the first function in the
stack trace, and if it can't then it assumes that there's no stack frame and
can end up walking up the stack incorrectly.

This change will output all the memory regions from the minidump.  Note that
AddDataToMapping() already handles overlapping regions, so this should only
result in the one extra region.

BUG=chromium-os:32626
TEST=Ran unittests