For devices which are not treble enabled, return the system shell.

For treble enabled devices, still return the appropriate shell depending
on whether the process is a vendor process or a system one.

Test: Manual testing: on a bullhead device, ran test programs from
      /vendor/bin which used popen() and system(). The calls succeeded.

Bug: 65054230

Bug: 64516799

Change-Id: I15dfdbb107cfca7c0f92f337c9bb46b9876eb38e
diff --git a/libc/Android.bp b/libc/Android.bp
index b5229cb..a0d1f23 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1509,7 +1509,11 @@
             srcs: ["bionic/mmap.cpp"],
         },
     },
-
+    product_variables: {
+        treble: {
+            cflags: ["-D__ANDROID_TREBLE__"],
+        },
+    },
     cppflags: ["-Wold-style-cast"],
     local_include_dirs: ["stdio"],
     include_dirs: ["bionic/libstdc++/include"],