scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

 # This file adds the functions to build the GCC test suite

 # Copyright 2010 DoréDevelopment

 # Created by Martin Lund <mgl@doredevelopment.dk>

 # Licensed under the GPL v2. See COPYING in the root of this package

 do_test_suite_gcc_get() { :; }

 do_test_suite_gcc_extract() { :; }

 do_test_suite_gcc_build() { :; }

 # Overide functions depending on configuration

 if [ "${CT_TEST_SUITE_GCC}" = "y" ]; then

 do_test_suite_gcc_build() {

     CT_DoStep INFO "Installing GCC test suite"

     CT_DoExecLog ALL mkdir -p "${CT_TEST_SUITE_DIR}/gcc"

     CT_DoExecLog ALL cp -a "${CT_LIB_DIR}/contrib/gcc-test-suite/default.cfg"               \

                            "${CT_LIB_DIR}/contrib/gcc-test-suite/Makefile"                  \

                            "${CT_LIB_DIR}/contrib/gcc-test-suite/README"                    \

                            "${CT_SRC_DIR}/gcc-${CT_CC_VERSION}/gcc/testsuite"               \

                            "${CT_TEST_SUITE_DIR}/gcc"

     CT_DoExecLog ALL sed -i -r -e "s/@@DG_TARGET@@/${CT_TARGET}/g;"     \

                          "${CT_TEST_SUITE_DIR}/gcc/Makefile"

     CT_EndStep

 }

 fi # CT_TEST_SUITE_GCC