arm64: defconfig: Disable tracing on genericarmv8-64

Ftracing has been seen to take up a lot of memory (~16 MB).
On certain environments where the amount of memory is scarce,
this is not ideal as it can contribute to memory pressure,
so disable it for genericarmv8-64 for the time being.

Change-Id: I9a8cc66e70caadc53a5cf63a39842c9548fddf25
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>