Switch libunwind_llvm over to building for NDK 21.

It would be preferable if this could build for API level 9, but it
depends on dl_iterate_phdr(), which only appears in API level 21 for
ARM.

Change-Id: I769a3ee19bf4a07cd75987251f2708b1f293407c
diff --git a/Android.bp b/Android.bp
index 1c89df4..a256b4f 100644
--- a/Android.bp
+++ b/Android.bp
@@ -45,6 +45,7 @@
 cc_library_static {
     name: "libunwind_llvm",
     clang: true,
+    sdk_version: "21",
     srcs: LIBUNWIND_SRC_FILES,
     include_dirs: LIBUNWIND_INCLUDES,
     local_include_dirs: LIBUNWIND_LOCAL_INCLUDES,