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>

 The m68k mcpu processor does not like unaligned access

 Disable at least mcpu32, m68010 and m68020. These processors certainly

 do not like unaligned accesses.

 Signed-off-by: Remy Bohmer <linux@bohmer.net>

 ---

  newlib/libc/machine/m68k/memcpy.S |    2 +-

  1 file changed, 1 insertion(+), 1 deletion(-)

 Index: newlib-1.18.0/newlib/libc/machine/m68k/memcpy.S

 ===================================================================

 --- newlib-1.18.0.orig/newlib/libc/machine/m68k/memcpy.S

 +++ newlib-1.18.0/newlib/libc/machine/m68k/memcpy.S

 @@ -15,7 +15,7 @@

  #include "m68kasm.h"

 -#if defined (__mcoldfire__) || defined (__mcpu32__) || defined (__mc68010__) || defined (__mc68020__) || defined (__mc68030__) || defined (__mc68040__) || defined (__mc68060__)

 +#if defined (__mcoldfire__) || defined (__mc68030__) || defined (__mc68040__) || defined (__mc68060__)

  # define MISALIGNED_OK 1

  #else

  # define MISALIGNED_OK 0