gdb_test_multiple: Fix user code argument processing

gdb_test_multiple: Fix user code argument processing

While teaching gdb_test_multiple to forward "-i" to gdb_expect, I
found that with:

      gdb_test_multiple (...) {
        -i $some_variable -re "..." {}
      }

$some_variable was not getting expanded in the gdb_test_multiple
caller's scope.  This is a bug inside gdb_test_multiple.  When
processing an argument in passed in user code, it was appending the
original argument literally, instead of appending the uplist'ed
argument.

gdb/testsuite/ChangeLog:
2015-04-07  Pedro Alves  <palves@redhat.com>

        * lib/gdb.exp (gdb_test_multiple): When processing an argument,
	append the substituted item, not the original item.