Expose libcompiler_rt-extras to the NDK.

The NDK can't fully switch to compiler-rt until the unwinders have
been sorted out, so it needs this for the time being.

Test: build/soong/scripts/build-ndk-prebuilts.sh
Bug: android/ndk#294
Change-Id: I65516f4369f4e6c39b54e3c049df224b8d908d31

Android.bp

@@ -22,8 +22,9 @@
 cc_library_static {
     name: "libcompiler_rt-extras",
     host_supported: true,
+    static_ndk_lib: true,
     vendor_available: true,
-    sdk_version: "9",
+    sdk_version: "minimum",
     // The following list contains functions that are not available in libgcc.a, so
     // we potentially need them when using a Clang-built component (e.g., -ftrapv
     // with 64-bit integer multiplies. See http://llvm.org/bugs/show_bug.cgi?id=14469.)