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.

Bug: http://b/153025717
Test: device boots, manually inspect some linker command lines
Change-Id: Icc79844f5e70f0eaa97ea758449c30fbddd030d2