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 --git a/mpn/generic/toom44_mul.c b/mpn/generic/toom44_mul.c

 --- a/mpn/generic/toom44_mul.c

 +++ b/mpn/generic/toom44_mul.c

 @@ -116,17 +116,16 @@

    TMP_MARK;

 -  as1  = TMP_SALLOC_LIMBS (n + 1);

 -  asm1 = TMP_SALLOC_LIMBS (n + 1);

 -  as2  = TMP_SALLOC_LIMBS (n + 1);

 -  ash  = TMP_SALLOC_LIMBS (n + 1);

 -  asmh = TMP_SALLOC_LIMBS (n + 1);

 -

 -  bs1  = TMP_SALLOC_LIMBS (n + 1);

 -  bsm1 = TMP_SALLOC_LIMBS (n + 1);

 -  bs2  = TMP_SALLOC_LIMBS (n + 1);

 -  bsh  = TMP_SALLOC_LIMBS (n + 1);

 -  bsmh = TMP_SALLOC_LIMBS (n + 1);

 +  as1  = TMP_ALLOC_LIMBS (10 * n + 10);

 +  asm1 = as1  + n + 1;

 +  as2  = asm1 + n + 1;

 +  ash  = as2  + n + 1;

 +  asmh = ash  + n + 1;

 +  bs1  = asmh + n + 1;

 +  bsm1 = bs1  + n + 1;

 +  bs2  = bsm1 + n + 1;

 +  bsh  = bs2  + n + 1;

 +  bsmh = bsh  + n + 1;

    gp = pp;

    hp = pp + n + 1;