configure: AC_PATH_TARGET_TOOL for LD

We want to make sure that LD is set to an absolute path in order to be
consistent with the `LD=$(command -v ld)` call. The AC_PATH_TARGET_TOOL
macro uses the absolute path rather than AC_CHECK_TARGET_TOOL which
might use a relative path.

m4/find_merge_objects.m4

@@ -45,7 +45,7 @@ AC_DEFUN([FIND_MERGE_OBJECTS],[
       if test "$result" = "1"; then
           AC_MSG_NOTICE([$MergeObjsCmd is broken due to binutils 22266, looking for another linker...])
           MergeObjsCmd=""
-          AC_CHECK_TARGET_TOOL([MergeObjsCmd], [ld])
+          AC_PATH_TARGET_TOOL([MergeObjsCmd], [ld])
           CHECK_FOR_GOLD_T22266($MergeObjsCmd)
           if test "$result" = "1"; then
               AC_MSG_ERROR([Linker is affected by binutils 22266 but couldn't find another unaffected linker. Please set the MergeObjsCmd variable to a functional linker.])