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>

 YEM-20070519:

 bits/sysnum.h needs a cross compiler to be built. Fortunately, this

 header is not needed to build gcc. Move generation of this header.

 Index: uClibc/Makefile.in

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

 --- uClibc/Makefile.in	(revision 18651)

 +++ uClibc/Makefile.in	(working copy)

 @@ -91,6 +91,11 @@

  			$(LN) -fs $$i .; \

  		done; \

  	fi

 +ifeq ($(UCLIBC_HAS_LOCALE),y)

 +	$(MAKE) locale_headers

 +endif

 +

 +pregen: headers

  	$(Q)\

  	set -e; \

  	cd $(top_builddir); \

 @@ -102,12 +107,7 @@

  	else \

  		mv -f $$tmp include/bits/sysnum.h; \

  	fi

 -ifeq ($(UCLIBC_HAS_LOCALE),y)

 -	$(MAKE) locale_headers

 -endif

 -pregen: headers

 -

  install: install_runtime install_dev