If available, use kernel's save_stack_trace_regs() for fallback stack unwind

If available, use kernel's save_stack_trace_regs() for fallback stack unwind

When the dwarf information is unavailable SystemTap resorts to
alterntive mechanisms to unwinds the stack. Only the x86 kernels have
unwind_start(), unwind_done(), and unwind_get_return_address().  A
number of architectures including the s390, powerpc, and openrisc
currently export save_stack_trace_reg() and expect that this will be
available on additional architectures such as arm64 in the future.
Adding a test to check for the export of the function and use it where
possible.