Only clear system_shared_libs when building for bionic

arm-optimized-routines is included in bionic, so it sets
system_shared_libs to empty to avoid a circular dependency.
Clear system_shared_libs only when compiling for bionic so that
the host tests can compile against musl.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I6915c45bb271683f04c2a5cbf9cbcfa2e332faba
diff --git a/Android.bp b/Android.bp
index ba814eb..8fe656e 100755
--- a/Android.bp
+++ b/Android.bp
@@ -56,10 +56,14 @@
     ],
 
     stl: "none",
-    static: {
-        system_shared_libs: [],
+    target: {
+        bionic: {
+            static: {
+                system_shared_libs: [],
+            },
+            header_libs: ["libc_headers"],
+        },
     },
-    header_libs: ["libc_headers"],
 }
 
 cc_library_static {