BACKPORT: ARM: wire up getrandom syscall

Clean cherry pick of eb645253.

Add the new getrandom syscall for ARM.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Bug: http://b/29621447
Change-Id: I6d50b57f3a61fbf9102c69103b9a5b7ebf239860
(cherry picked from commit eb645253)