scripts: move backtrace marker to CT_WORK_DIR

Avoid error when commands in scripts/crosstool-NG.sh fail
before CT_BUILD_DIR is set.

So we need to remove the backtrace marker of a potential previous
build. Previously, it was implicitly removed because we did remove
the directory it was in, which is no longer the case.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[yann.morin.1998@free.fr: remove backtrace marker on start of build]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <20121015094615.GA18673@sig21.net>
Patchwork-Id: 191498

 --- glibc-2.7/glibc-ports-2.7.orig/sysdeps/unix/sysv/linux/arm/nptl/sysdep-cancel.h	2007-06-06 19:27:04.000000000 +0200

 +++ glibc-2.7/glibc-ports-2.7/sysdeps/unix/sysv/linux/arm/nptl/sysdep-cancel.h	2008-05-18 16:57:21.000000000 +0200

 @@ -73,6 +73,9 @@

  # define DOCARGS_5	DOCARGS_4

  # define UNDOCARGS_5	UNDOCARGS_4

 +# define DOCARGS_6    DOCARGS_5

 +# define UNDOCARGS_6  UNDOCARGS_5

 +

  # ifdef IS_IN_libpthread

  #  define CENABLE	bl PLTJMP(__pthread_enable_asynccancel)

  #  define CDISABLE	bl PLTJMP(__pthread_disable_asynccancel)