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 -p -c -u -r1.196 elf32-arm.c

 diff -durN binutils-2.20.1.orig/bfd/elf32-arm.c binutils-2.20.1/bfd/elf32-arm.c

 --- binutils-2.20.1.orig/bfd/elf32-arm.c	2010-02-22 09:06:48.000000000 +0100

 +++ binutils-2.20.1/bfd/elf32-arm.c	2010-08-17 19:32:25.000000000 +0200

 @@ -3194,11 +3194,15 @@

  	  /* We have an extra 2-bytes reach because of

  	     the mode change (bit 24 (H) of BLX encoding).  */

 +	  /* A stub is needed only if this call is not throught a PLT

 +	     entry, because PLT stubs handle mode switching

 +	     already.  */

  	  if (branch_offset > (ARM_MAX_FWD_BRANCH_OFFSET + 2)

  	      || (branch_offset < ARM_MAX_BWD_BRANCH_OFFSET)

 -	      || ((r_type == R_ARM_CALL) && !globals->use_blx)

 -	      || (r_type == R_ARM_JUMP24)

 -	      || (r_type == R_ARM_PLT32))

 +	      || ( (((r_type == R_ARM_CALL) && !globals->use_blx)

 +		    || (r_type == R_ARM_JUMP24)

 +		    || (r_type == R_ARM_PLT32))

 +		   && !use_plt))

  	    {

  	      stub_type = (info->shared | globals->pic_veneer)

  		/* PIC stubs.  */