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)

 diff -durN gcc-4.4.0.orig/gcc/builtins.c gcc-4.4.0/gcc/builtins.c

 --- gcc-4.4.0.orig/gcc/builtins.c	2009-03-30 19:42:27.000000000 +0200

 +++ gcc-4.4.0/gcc/builtins.c	2009-05-27 21:38:01.000000000 +0200

 @@ -5768,6 +5768,9 @@

    trampolines_created = 1;

    INITIALIZE_TRAMPOLINE (r_tramp, r_func, r_chain);

 +  if (warn_trampolines)

 +    warning (OPT_Wtrampolines, "generating trampoline in object (requires executable stack)");

 +

    return const0_rtx;

  }

 diff -durN gcc-4.4.0.orig/gcc/common.opt gcc-4.4.0/gcc/common.opt

 --- gcc-4.4.0.orig/gcc/common.opt	2009-03-28 18:28:45.000000000 +0100

 +++ gcc-4.4.0/gcc/common.opt	2009-05-27 21:38:01.000000000 +0200

 @@ -197,6 +197,10 @@

  Common Var(warn_type_limits) Init(-1) Warning

  Warn if a comparison is always true or always false due to the limited range of the data type

 +Wtrampolines

 +Common Var(warn_trampolines) Init(1)

 +Warn whenever a trampoline is generated

 +

  Wuninitialized

  Common Var(warn_uninitialized) Warning

  Warn about uninitialized automatic variables