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)

# MIPS specific options for gcc

config CC_GCC_HAS_ARCH_OPTIONS

    default y

config CC_GCC_mips_llsc

    tristate

    prompt "Use llsc"

    default m

    help

      This configures how gcc will emit load-link, store-conditional and

      sync opcodes. Normally, you'd set this to 'M', and configure will do

      the correct thing (ie. enable llsc for Linux-based targets, disable

      for others). If you know better, you can force either using llsc or

      not using it (resp. 'Y' or 'N').

config CC_GCC_mips_synci

    tristate

    prompt "Use synci"

    default m

    help

      This configures how gcc will emit the synci opcode. Normally, you'd

      set this to 'M', and configure will do the correct thing (ie. disable).

      If you know better, you can force using synci (say 'Y'), or not using

      it (say 'N').

config CC_GCC_mips_plt

    bool

    prompt "Use copy relocations & PLTs"

    help

      This configures gcc to use copy relocations & PLTs. These are

      extensions to the traditional SVR4-based MIPS ABIs and require

      support from GNU binutils and the runtime C library.

      Say 'N' to keep the default and not use them, say 'Y' to use them.