Make apex availability of some bionic libraries explicit

The libraries are
- libdl_static
- liblinker_main
- liblinker_malloc
- libsystemproperties

The availability to runtime apex was done implicitly using a baseline map in
build/soong/apex/apex.go. Make this explicit in Android.bp

Bug: 281077552
Test: m nothing
Change-Id: I029ae204f6cfef8c301a20b7c4294636b60b38be
diff --git a/libdl/Android.bp b/libdl/Android.bp
index 3bda856..c7d7cf3 100644
--- a/libdl/Android.bp
+++ b/libdl/Android.bp
@@ -47,6 +47,9 @@
     sanitize: {
         never: true,
     },
+    apex_available: [
+        "com.android.runtime",
+    ],
 }
 
 cc_library {