unwind: tweak inclusion ordering to work around GCC

This is a slightly convoluted workaround.  GCC does not support the
__has_feature extension of clang, and this results in some issues with
static_asserts.  config.h defines static_assert as a macro with a C-specific
trickery.  This then propagates into the C++ headers included after config.h,
which are used with C++11 mode, enabling constexpr constructors.  The macro'ed
static_assert does not get treated as the static_assert builtin, and will cause
an error due to a non-empty constexpr constructor.  Tweaking the include order
permits the use of libc++ headers to build libunwind with GCC on Linux.

llvm-svn: 228809