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 -durN gcc-4.4.5.orig/gcc/config/sh/lib1funcs.asm gcc-4.4.5/gcc/config/sh/lib1funcs.asm

--- gcc-4.4.5.orig/gcc/config/sh/lib1funcs.asm	2009-04-10 01:23:07.000000000 +0200

+++ gcc-4.4.5/gcc/config/sh/lib1funcs.asm	2010-10-09 23:04:22.000000000 +0200

@@ -30,6 +30,11 @@

 !! recoded in assembly by Toshiyasu Morita

 !! tm@netcom.com

+#if defined(__ELF__) && defined(__linux__)

+.section .note.GNU-stack,"",%progbits

+.previous

+#endif

+

 /* SH2 optimizations for ___ashrsi3, ___ashlsi3, ___lshrsi3 and

    ELF local label prefixes by J"orn Rennecke

    amylaar@cygnus.com  */

diff -durN gcc-4.4.5.orig/gcc/config/sh/linux-atomic.asm gcc-4.4.5/gcc/config/sh/linux-atomic.asm

--- gcc-4.4.5.orig/gcc/config/sh/linux-atomic.asm	2009-04-10 01:23:07.000000000 +0200

+++ gcc-4.4.5/gcc/config/sh/linux-atomic.asm	2010-10-09 23:04:22.000000000 +0200

@@ -136,3 +136,6 @@

 ATOMIC_FETCH_AND_COMBOP(nand,and,not,4,l,mov)

 #endif /* ! __SH5__ */

+

+.section .note.GNU-stack,"",%progbits

+.previous