[libunwind] Drop -Wl,-z,defs for SHARED builds.

[plotfi]

Summary

Drop -Wl,-z,defs in shared builds of libunwind since during builds of ELF libraries, symbol resolution is incomplete.

[compnerd]

Can you please write a more complete commit message explaining the need for this.

As an aside for the review itself, could you please enumerate what symbols are unresolved and why they are unresolved.

[plotfi]

In D69902#1735678, @compnerd wrote:

Can you please write a more complete commit message explaining the need for this.

As an aside for the review itself, could you please enumerate what symbols are unresolved and why they are unresolved.

Sure.

[plotfi]

The symbols/errors are:

the vtable symbol may be undefined because the class is missing its key function (see https://lld.llvm.org/missingkeyfunction)

>>> referenced by libunwind.cpp: libunwind/src/CMakeFiles/unwind_shared.dir/libunwind.cpp.o:(typeinfo for libunwind::AbstractUnwindCursor)
ld.lld: error: undefined symbol: vtable for __cxxabiv1::__si_class_type_info
>>> referenced by libunwind.cpp: libunwind/src/CMakeFiles/unwind_shared.dir/libunwind.cpp.o:(typeinfo for libunwind::UnwindCursor<libunwind::LocalAddressSpace, libunwind::Registers_arm>)

[plotfi]

Abandoning: We are going to try building libunwind without rtti.

libunwind/src/CMakeLists.txt

set_target_properties(unwind_shared
                           LINK_FLAGS
                             "${LIBUNWIND_LINK_FLAGS}"
                           OUTPUT_NAME
                             "unwind"
                           VERSION
                             "1.0"
                           SOVERSION
                             "1")
+  string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
   list(APPEND LIBUNWIND_BUILD_TARGETS "unwind_shared")
   if (LIBUNWIND_INSTALL_SHARED_LIBRARY)
     list(APPEND LIBUNWIND_INSTALL_TARGETS "unwind_shared")
   endif()
 endif()
 
 # Build the static library.
 if (LIBUNWIND_ENABLE_STATIC)