stdlib: Fix incorrect path and checks for DRAMsim3

There are three bugs fixed in this patch:

1. The `dram_3_dir` was missing the "dramsim3" directory.
2. Missing `not` when checking if configs is a directory.
3. Missing `not` when checking if input file is a file.

Change-Id: I185f4832c1c2f1ecc4e138c148ad7969ef9b6fd4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/71058
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed