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>

# windows config options

## depends on EXPERIMENTAL && ARCH_x86

##

## select WINDOWS

##

## help Build a toolchain targeting systems running Windows as host