scripts: add option to start an interactive debug shell

Add an option that, when a command fails:
- starts an interactive shell with the failed command's environment
- attempts re-execution of the failed command, continues, or aborts
at user's whim.

Before starting the debug-shell, the backtrace is printed.
When exiting for an abort, the standard error message is printed.

Based on an idea and a patch from: Johannes Stezenbach <js@sig21.net>
http://sourceware.org/ml/crossgcc/2012-09/msg00144.html

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[yann.morin.1998@free.fr: integrate in the fault handler]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Johannes Stezenbach <js@sig21.net>
Patchwork-Id: 191571
Patchwork-Id: 191668

diff -durN gcc-3.4.6.orig/gcc/config/sh/sh.h gcc-3.4.6/gcc/config/sh/sh.h

--- gcc-3.4.6.orig/gcc/config/sh/sh.h	2006-01-12 02:20:32.000000000 +0100

+++ gcc-3.4.6/gcc/config/sh/sh.h	2007-08-15 23:03:09.000000000 +0200

@@ -422,6 +422,10 @@

 do {									\

   if (LEVEL)								\

     flag_omit_frame_pointer = -1;					\

+  if (LEVEL <= 2)							\

+    {									\

+      flag_reorder_blocks = 0;						\

+    }									\

   if (SIZE)								\

     target_flags |= SPACE_BIT;						\

   if (TARGET_SHMEDIA && LEVEL > 1)					\