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