arm/arm64: Export save_stack_trace_tsk()

The kernel watchdog is a great debugging tool for finding tasks that
consume a disproportionate amount of CPU time in contiguous chunks. One
can imagine building a similar watchdog for arbitrary driver threads
using save_stack_trace_tsk() and print_stack_trace(). However, this is
not viable for dynamically loaded driver modules on ARM platforms
because save_stack_trace_tsk() is not exported for those architectures.
Export save_stack_trace_tsk() for the ARM64 architecture to align with
x86 and support various debugging use cases such as arbitrary driver
thread watchdog timers.

Change-Id: I61e9d2afc4703a786fa6dcaf82fe46c0ed250045
CRs-Fixed: 2061326
Signed-off-by: Dustin Brown <dustinb@codeaurora.org>