testsuite: add print-stack.exp

I wrote this support file to help me debug Tcl issues in the
testsuite.

Adding a call to:

  print_stack_backtrace

somewhere in a .exp file (along with "load_lib print-stack.exp") leads
to the interpreter printing a backtrace in a form that e.g. Emacs can
consume, with filename:linenum: lines, and quoting the line of .exp
source code.

Fer example, adding a print_stack_backtrace to scansarif.exp in
run-sarif-pytest I get this output:

VVV START OF BACKTRACE VVV
  /home/david/coding/gcc-newgit/src/gcc/testsuite/lib/scansarif.exp:142: frame 16 in proc print_stack_backtrace
    142 |     print_stack_backtrace
  <proc>: frame 15 in proc run-sarif-pytest
  <eval>: frame 14 in proc dg-final-proc
  /usr/share/dejagnu/dg.exp:851: frame 13 in proc dg-final-proc
    851 | 	if {[catch "dg-final-proc $prog" errmsg]} {
  <eval>: frame 12 in proc saved-dg-test
  /home/david/coding/gcc-newgit/src/gcc/testsuite/lib/gcc-dg.exp:1080: frame 11 in proc saved-dg-test
    1080 | 	if { [ catch { eval saved-dg-test $args } errmsg ] } {
  /usr/share/dejagnu/dg.exp:559: frame 10 in proc dg-test
    559 | 	dg-test $testcase $options ${default-extra-options}
  /home/david/coding/gcc-newgit/src/gcc/testsuite/gcc.dg/sarif-output/sarif-output.exp:28: frame 9
    28 | dg-runtest [lsort [glob -nocomplain $srcdir/$subdir/*.c]] "" ""
  <eval>: frame 8
  <eval>: frame 7
  /usr/share/dejagnu/runtest.exp:1460: frame 6
    1460 | 	if { [catch "uplevel #0 source $test_file_name"] == 1 } {
  /usr/share/dejagnu/runtest.exp:1886: frame 5 in proc dg-runtest
    1886 | 			runtest $test_name
  /usr/share/dejagnu/runtest.exp:1845: frame 4 in proc dg-runtest
    1845 | 		    foreach test_name [lsort [find ${dir} *.exp]] {
  /usr/share/dejagnu/runtest.exp:1788: frame 3 in proc dg-runtest
    1788 | 	    foreach dir "${test_top_dirs}" {
  /usr/share/dejagnu/runtest.exp:1669: frame 2 in proc dg-runtest
    1669 |     foreach pass $multipass {
  /usr/share/dejagnu/runtest.exp:1619: frame 1 in proc dg-runtest
    1619 | foreach current_target $target_list {
^^^  END OF BACKTRACE  ^^^

and can click on the lines in Emacs's compilation buffer to take
me to the relevant places.

I found this made it *much* easier to debug my .exp files.  That
said, I'm uncomfortable with Tcl, and so
(a) there may be a better way of doing this
(b) I may have made mistakes

gcc/testsuite/ChangeLog:
	* lib/print-stack.exp: New file.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>