Add basic tests for <link.h>.

The dl_iterate_phdr test is new, but the exidx test is largely copy & paste
from ndk_translation.

Bug: http://b/31556066
Test: ran tests
Change-Id: If754f1459ef1d789ee98a89cd2215188edca51da
diff --git a/tests/Android.bp b/tests/Android.bp
index ae8d7fc..c936c5c 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -256,6 +256,7 @@
         "dl_test.cpp",
         "dlfcn_symlink_support.cpp",
         "dlfcn_test.cpp",
+        "link_test.cpp",
         "pthread_dlfcn_test.cpp",
     ],
     static_libs: [