stdlib,configs: Config added for RISCV boot-test using stdlib.

This change adds a config file to boot Ubuntu-20.04 using RISCV ISA
using gem5 stdlib. It also adds a new test for the same.

Signed-off-by: Kaustav Goswami <kggoswami@ucdavis.edu>
Change-Id: Id2b5e46e7ba9e3c4701c97330406537dffa44479
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/53024
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/gem5/gem5_library_example_tests/test_gem5_library_examples.py

@@ -157,3 +157,20 @@ if os.access("/dev/kvm", mode=os.R_OK | os.W_OK):
         valid_hosts=constants.supported_hosts,
         length=constants.long_tag,
     )
+
+gem5_verify_config(
+    name="test-gem5-library-example-riscv-ubuntu-run",
+    fixtures=(),
+    verifiers=(),
+    config=joinpath(
+        config.base_dir,
+        "configs",
+        "example",
+        "gem5_library",
+        "riscv-ubuntu-run.py",
+    ),
+    config_args=[],
+    valid_isas=(constants.riscv_tag,),
+    valid_hosts=constants.supported_hosts,
+    length=constants.long_tag,
+)