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)

 # Compute sh-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH_SH_VARIANT}${target_endian_eb}"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

     # Endianness stuff

     case "${CT_ARCH_BE},${CT_ARCH_LE}" in

         y,) CT_ARCH_ENDIAN_CFLAG=-mb;;

         ,y) CT_ARCH_ENDIAN_CFLAG=-ml;;

     esac

     # CFLAGS

     case "${CT_ARCH_SH_VARIANT}" in

         sh3)    CT_ARCH_ARCH_CFLAG=-m3;;

         sh4*)

             case "${CT_ARCH_FLOAT_HW},${CT_ARCH_FLOAT_SW}" in

                 y,) CT_ARCH_ARCH_CFLAG="-m4${CT_ARCH_SH_VARIANT##sh?}";;

                 ,y) CT_ARCH_ARCH_CFLAG="-m4${CT_ARCH_SH_VARIANT##sh?}-nofpu";;

             esac

             ;;

     esac

     CT_ARCH_FLOAT_CFLAG=

 }