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.
(cherry picked from commit 15142f793a)
Change-Id: Ide73171fc4e513b39bee74e2270252c3b32e23cd
f893e743b6