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 -uNpr gcc-4.2.1_orig/gcc/config/rs6000/darwin-ldouble.c gcc-4.2.1/gcc/config/rs6000/darwin-ldouble.c

 --- gcc-4.2.1_orig/gcc/config/rs6000/darwin-ldouble.c	2007-03-05 11:54:00.000000000 -0500

 +++ gcc-4.2.1/gcc/config/rs6000/darwin-ldouble.c	2008-01-31 17:51:24.000000000 -0500

 @@ -70,6 +70,8 @@ Software Foundation, 51 Franklin Street,

     but GCC currently generates poor code when a union is used to turn

     a long double into a pair of doubles.  */

 +#if defined (_SOFT_FLOAT) && defined (__LONG_DOUBLE_128__)

 +

  long double __gcc_qadd (double, double, double, double);

  long double __gcc_qsub (double, double, double, double);

  long double __gcc_qmul (double, double, double, double);

 @@ -219,8 +221,6 @@ __gcc_qdiv (double a, double b, double c

    return z.ldval;

  }

 -#if defined (_SOFT_FLOAT) && defined (__LONG_DOUBLE_128__)

 -

  long double __gcc_qneg (double, double);

  int __gcc_qeq (double, double, double, double);

  int __gcc_qne (double, double, double, double);