Left the default symbol.ARCH value intact and changed stack_core instead
so that it will assume the ABI is arm until it sees an ABI line. This
allows compatibility for people who are used to pasting portions of a
tombstone instead of the whole thing (assuming said tombstone is arm)
while additionally supporting other architectures if a whole tombstone
is pasted in.
Change-Id: Ib8b5754920d2e2aa5017c29daeca59abcf4b547d
15142f793a