After r364423, which ensures the callbacks that dl_iterate_phdr(3)
performs are protected by an exclusive lock, even for statically linked
programs, it is safe to re-enable libunwind's FrameHeaderCache, which I
temporarily disabled in r364263.

Meanwhile upstream has also used the _LIBUNWIND_USE_FRAME_HEADER_CACHE
for this purpose, so the only thing needed is to add this as a
compile-time command line flag.

While here, reformat the CFLAGS lines a little bit.

MFC after: 6 weeks
X-MFC-With: r364284, r364423