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

core/binary.mk

@@ -266,10 +266,7 @@ ifneq ($(LOCAL_SDK_VERSION),)
       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.

core/cxx_stl_setup.mk

@@ -82,15 +82,7 @@ ifneq ($(filter $(my_cxx_stl),libc++ libc++_static),)
         endif
     endif
 else ifeq ($(my_cxx_stl),ndk)
-    # Using an NDK STL. Handled in binary.mk, except for the unwinder.
-    # TODO: Switch the NDK over to the LLVM unwinder for non-arm32 architectures.
-    ifeq (arm,$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH))
-        my_static_libraries += libunwind_llvm
-        my_ldflags += -Wl,--exclude-libs,libunwind_llvm.a
-    else
-        my_static_libraries += libgcc_stripped
-        my_ldflags += -Wl,--exclude-libs,libgcc_stripped.a
-    endif
+    # Using an NDK STL. Handled in binary.mk.
 else ifeq ($(my_cxx_stl),libstdc++)
     $(error $(LOCAL_PATH): $(LOCAL_MODULE): libstdc++ is not supported)
 else ifeq ($(my_cxx_stl),none)