Fix whitespace in soong_config_set calls

In make, soong_config_set uses an $(eval) to set it's value, expanding
the value of the soong config variable _before_ evaluating the value.
Because of this, make will strip trailing whitespace like it does on
regular assignments.

Make rbcrun match this behavior.

Test: ./out/rbcrun ./build/make/tests/run.rbc
Change-Id: I907e85cdf50f6fac54331c0d1044f0d53bec22ed
diff --git a/tests/run.rbc b/tests/run.rbc
index 33583eb..85d6c09 100644
--- a/tests/run.rbc
+++ b/tests/run.rbc
@@ -144,7 +144,8 @@
             "v2": "def"
         },
         "NS2": {
-            "v3": "xyz"
+            "v3": "xyz",
+            "v4": "xyz"
         }
     },
     {k:v for k, v in sorted(ns.items()) }