mem-ruby,configs: Enable Ruby with NULL build

After removing `get_runtime_isa`, the `send_evicts` function in the ruby
configs assumes that there is an ISA built. This change short-circuits
that logic if the current build is the NULL (none) ISA.

Change-Id: I75fefe3d70649b636b983c4d2145c63a9e1342f7
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>

configs/ruby/Ruby.py

@@ -48,6 +48,7 @@ from m5.util import (
 )
 
 from gem5.isas import ISA
+from gem5.runtime import get_supported_isas
 
 addToPath("../")
 
@@ -330,6 +331,8 @@ def send_evicts(options):
     # 1. The O3 model must keep the LSQ coherent with the caches
     # 2. The x86 mwait instruction is built on top of coherence invalidations
     # 3. The local exclusive monitor in ARM systems
+    if get_supported_isas() == {ISA.NULL}:
+        return False
 
     if (
         hasattr(m5.objects, "DerivO3CPU")