scripts/xldd: use user's sed and grep

xldd uses sed and grep as detected by ./configure. This works well if is
used on the machine that build the toolchain.

But if the user moves the toolchain to another machine where sed and grep
are not in the same directory (eg. /bin/sed vs. /usr/bin/sed), then xldd
will stop functionning.

Fix that by using ${SED} and ${GREP} if they are set in the environment.

Reported-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 diff -durN glibc-2.7.orig/Makerules glibc-2.7/Makerules

 --- glibc-2.7.orig/Makerules	2007-08-26 04:18:03.000000000 +0200

 +++ glibc-2.7/Makerules	2008-07-28 15:17:27.000000000 +0200

 @@ -1203,6 +1203,9 @@

  # The include magic above causes those files to use this variable for flags.

  CPPFLAGS-nonlib = -DNOT_IN_libc=1

 +ifneq ($(OBJDUMP_FOR_HOST),)

 +OBJDUMP = $(OBJDUMP_FOR_HOST)

 +endif

  ifeq ($(versioning),yes)

  # Generate normalized lists of symbols, versions, and data sizes.