Set sys.platform value for hermetic Python.

sys.platform is determined at compile/build time. By default, CPython
will give it a "unknown" value if we didn't set the cflag.

Test: adb push device_test /data, adb shell, ./device_test
Bug: b/70218680
Change-Id: If5d914d3c43b1cfd3df1e57c81becacf2ac59e36
diff --git a/Android.bp b/Android.bp
index 8b78e0b..4a4d6e2 100644
--- a/Android.bp
+++ b/Android.bp
@@ -152,6 +152,18 @@
 cc_library_static {
     name: "py2-interp-python",
     defaults: ["py2-interp-defaults"],
+    target: {
+        linux: {
+            cflags: [
+                "-DPLATFORM=\"linux2\"",
+            ],
+        },
+        darwin_x86_64: {
+            cflags: [
+                "-DPLATFORM=\"darwin\"",
+            ],
+        },
+    },
     srcs: [
         "Python/_warnings.c",
         "Python/Python-ast.c",
@@ -315,7 +327,7 @@
         android: {
             shared_libs: [
                 "libsqlite",
-            ]
+            ],
         },
     },
 }