LU-12323 libcfs: check if save_stack_trace_tsk is exported

LU-12323 libcfs: check if save_stack_trace_tsk is exported

Lustre 2.12 commit afedf9343686504c89f2e28cf6133540166f2347 introduced
the use of save_stack_trace_tsk, but this symbol is not exported for
all architectures. When it's possible we can use save_stack_trace
instead. Otherwise skip printing stack trace.

Cray-bug-id: LUS-7352
Test-Parameters: clientarch=aarch64
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I142b542f5c5672abbad461a621aedd1e49db1bdd
Reviewed-on: https://review.whamcloud.com/34937
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>