libebl [3/12]: eu-stacktrace: use new register handling api

libebl [3/12]: eu-stacktrace: use new register handling api

Dummy commit to show how the sample_set_initial_registers callback in
eu-stacktrace would use the proper libebl
ebl_set_initial_registers_sample function (if it were public).

* src/Makefile.am (stacktrace_LDADD): Add libebl.
* src/stacktrace.c (sample_registers_cb): New function,
  though identical to pid_thread_state_registers_cb.
  (sample_set_initial_registers): (XXX Invoke
  ebl_set_initial_registers_sample instead of containing
  platform-specific code directly.  This is now commented out.
  Patch12 in the series replaces with code in
  libdwfl_stacktrace/dwflst_perf_frame.c.)

Signed-off-by: Serhei Makarov <serhei@serhei.io>