buildgcc: Fix msys2 crossgcc build fail

A leading double slash can result when $DESTDIR/$TARGETDIR is expanded
in the libelf portion of buildgcc. The leading double slash causes buildgcc to fail when run from Windows/Msys2. Replace $DESTDIR/$TARGETDIR
with $DESTDIR$TARGETDIR to avoid the problem.

Change-Id: Ide2bae41c07c1566f80104c3a2e2acab53de0d17
Signed-off-by: Scott Duplichan <scott@notabs.org>
Reviewed-on: http://review.coreboot.org/7788
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index 7f2643f..a35bf33 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -437,7 +437,7 @@
 	../${LIBELF_DIR}/configure --disable-shared --prefix=$TARGETDIR \
 		--infodir=$TARGETDIR/info CFLAGS="$HOSTCFLAGS" || touch .failed
 	$MAKE $JOBS || touch .failed
-	$MAKE install prefix=$DESTDIR/$TARGETDIR || touch .failed
+	$MAKE install prefix=$DESTDIR$TARGETDIR || touch .failed
 
 	normalize_dirs