Switch platform-NDK unwinder to LLVM libunwind.a prebuilt

Link against the libunwind.a shipped with the NDK so it matches what's
linked into the NDK's libc++_shared.so.

For arm32, binary.mk and cxx_stl_setup.mk had been adding two different
versions of the LLVM unwinder to the same linker command-line. Remove
the duplicate one from cxx_stl_setup.mk.

-Wl,--exclude-libs is now unnecessary because the symbols are marked
hidden in the archive itself.

Bug: http://b/153025717
Test: device boots, manually inspect some linker command lines
Change-Id: Ie7182bd565014348b16448976dc6587d2885ba99
diff --git a/core/binary.mk b/core/binary.mk
index 2c20eed..0d7206f 100644
--- a/core/binary.mk
+++ b/core/binary.mk
@@ -266,10 +266,7 @@
       endif
     endif
 
-    ifneq (,$(filter armeabi armeabi-v7a,$(my_cpu_variant)))
-      my_ndk_stl_static_lib += $(my_libcxx_libdir)/libunwind.a
-    endif
-
+    my_ndk_stl_static_lib += $(my_libcxx_libdir)/libunwind.a
     my_ldlibs += -ldl
   else # LOCAL_NDK_STL_VARIANT must be none
     # Do nothing.