Mount tmpfs on /cores

Proper selinux labeling support for files on rootfs was
never completely implemented. Instead of putting coredump
files on rootfs, put them on tmpfs instead.

See: http://www.mail-archive.com/seandroid-list@tycho.nsa.gov/msg01815.html

Bug: 18227650
Change-Id: I2eeabee4fe1a14bfbf990a4a518d538d6b4b6e87