arm32 has two special APIs to find exidx exception handling info,
dl_unwind_find_exidx and __gnu_Unwind_Find_exdix. The two functions have
identical behavior and function prototypes. libgcc's arm32 unwinder calls
__gnu_Unwind_Find_exdix, whereas LLVM's libunwind previously called
__gnu_Unwind_Find_exdix, but switched to dl_unwind_find_exidx as a result
of three patches (D30306, D30681, D39468).
In Bionic, for dynamic linking, __gnu_Unwind_Find_exdix in libc.so calls
dl_unwind_find_exidx in libdl.so.
For static executables, though, __gnu_Unwind_Find_exdix in libc.a used the
__exidx_* symbols, while dl_unwind_find_exidx in libdl.a(libdl_static.o)
was a return-0 no-op.
To fix the LLVM unwinder, replace the no-op dl_unwind_find_exidx in
libdl.a with a real function in libc.a(exidx_static.o), and have the GNU
function call the dl function for more consistency with dynamic linking.
dl_iterate_phdr follows a similar pattern, where the function exists in
libc.a and libdl.so (not libc.so or libdl.a).
This change makes unwinding work with an updated libunwind_llvm on arm32,
and it helps to allow unwinding in static executables without libdl.a.
Bug: https://github.com/android/ndk/issues/1094
Bug: http://b/141485154
Test: NDK tests, bionic unit tests
Change-Id: Ieeeb9b39a0e28544e21f9afe6fe51ef10d7c828c
22a6a058c7