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)

 Original patch from: ../4.3.2/230-pr34571.patch

 -= BEGIN original header =-

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/64_all_gcc43-pr34571.patch

 http://gcc.gnu.org/PR34571

 2007-12-26  Rask Ingemann Lambertsen  <rask@sygehus.dk>

 	PR target/34571

 	* config/alpha/alpha.c (alpha_cannot_force_const_mem): Use

 	symbolic_operand.

 	* varasm.c (output_constant_pool_1): Fix typo.

 -= END original header =-

 diff -durN gcc-4.3.3.orig/gcc/config/alpha/alpha.c gcc-4.3.3/gcc/config/alpha/alpha.c

 --- gcc-4.3.3.orig/gcc/config/alpha/alpha.c	2008-09-09 01:16:55.000000000 +0200

 +++ gcc-4.3.3/gcc/config/alpha/alpha.c	2009-01-27 22:25:29.000000000 +0100

 @@ -1113,8 +1113,7 @@

  static bool

  alpha_cannot_force_const_mem (rtx x)

  {

 -  enum rtx_code code = GET_CODE (x);

 -  return code == SYMBOL_REF || code == LABEL_REF || code == CONST;

 +  return symbolic_operand (x, GET_MODE (x));

  }

  /* We do not allow indirect calls to be optimized into sibling calls, nor