Make libc.so global

This is a way to avoid loading multiple libc.so
when non-default namespace search path includes
/system/lib. This is used by some art tests.

Bug: http://b/26833548
Test: m -j32 test-art-target-run-test-004-JniTest
Change-Id: I919d3a0560bd3c9ac19df21a235641a667f0f017

libc/Android.bp

@@ -1665,6 +1665,8 @@ cc_library {
         keep_symbols: true,
     },
 
+    ldflags: ["-Wl,-z,global"],
+
     // Do not pack libc.so relocations; see http://b/20645321 for details.
     pack_relocations: false,