[libunwind] Replace chain-of-ifdefs for dl_iterate_phdr

Define a _LIBUNWIND_USE_DL_ITERATE_PHDR macro in config.h when there is
no other unwind info lookup method. Also define a
_LIBUNWIND_USE_DL_UNWIND_FIND_EXIDX macro to factor out
(__BIONIC__ and _LIBUNWIND_ARM_EHABI).

Differential Revision: https://reviews.llvm.org/D86768