Show the number of VMAs in the tombstone.

Tombstones (especially ones with lots of VMAs) are regularly truncated.
We can at least show the number of VMAs, though, for anyone interested
in knowing whether they got close to the default 64Ki limit.

Bug: http://b/66911122
Test: ran crasher, examined tombstone
Change-Id: I286db66f28f132307d573dbe5164efc969dc6ddc