fix failure to use add-cfi scripts on asm when building out-of-tree

use $srcdir in configure test for add-cfi script.

configure

@@ -409,7 +409,7 @@ test "$debug" = yes && CFLAGS_AUTO=-g
 #
 printf "checking whether we should preprocess assembly to add debugging information... "
 if fnmatch '-g*|*\ -g*' "$CFLAGS_AUTO $CFLAGS" &&
-   test -f "tools/add-cfi.$ARCH.awk" &&
+   test -f "$srcdir/tools/add-cfi.$ARCH.awk" &&
    printf ".file 1 \"srcfile.s\"\n.line 1\n.cfi_startproc\n.cfi_endproc" | $CC -g -x assembler -c -o /dev/null 2>/dev/null -
 then
   ADD_CFI=yes