ld testsuite: Sanitize output from ld

ld testsuite: Sanitize output from ld

Leading dashes from an LD error message confuse send_log, because the
string is taken to be an option.

This patch fixes the following DejaGnu error:

  --defsym:1: undefined symbol `foo2' referenced in expression
  failed with: <--defsym:1: undefined symbol `foo2' referenced in expression>, no expected output
  ERROR: tcl error sourcing .../ld/testsuite/ld-gc/gc.exp.
  ERROR: usage: send [args] string
      while executing
  "send_log "$comp_output\n""
      (procedure "run_dump_test" line 376)

	* testsuite/lib/ld-lib.exp (run_dump_test): Pass -- to send_log.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>