[PATCH][dejagnu] truncate absolute file path into relative for dg-output

[PATCH][dejagnu] truncate absolute file path into relative for dg-output

  gcc/testsuite/
    * lib/gcc-dg.exp (${tool}_load): Truncate gcc output.
    * lib/prune.exp (prune_gcc_output): New absolute path to relative path
    truncation pattern.

From-SVN: r217408