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

CT_EXPERIMENTAL=y

CT_LOCAL_TARBALLS_DIR="${HOME}/src"

CT_SAVE_TARBALLS=y

CT_LOG_EXTRA=y

CT_ARCH_mips=y

CT_TARGET_VENDOR="malta"

CT_KERNEL_linux=y

CT_KERNEL_V_2_6_38_8=y

CT_BINUTILS_V_2_21_1a=y

CT_CC_V_4_4_6=y

# CT_CC_GCC_USE_GRAPHITE is not set

CT_LIBC_EGLIBC_V_2_13=y

CT_DEBUG_gdb=y

# CT_GDB_GDBSERVER_STATIC is not set

CT_GDB_V_7_2a=y

CT_GMP_V_5_0_1=y

CT_MPFR_V_3_0_0=y

CT_TEST_SUITE_GCC=y