[libcxxabi] Automatically use static libunwind when required

[smeenai]

Summary

If we attempt to use unwind_shared when LIBUNWIND_ENABLE_SHARED is OFF,
we'll get link errors. LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY
avoids that, but it seems redundant to have to specify it manually.
Automatically switch libunwind statically when its shared build is
disabled, which also matches compiler-rt [1].

[1] https://github.com/llvm/llvm-project/blob/71bfec762bd970e7834f58c158ddc15f93402d7a/compiler-rt/CMakeLists.txt#L238-L240

[phosek]

LGTM

libcxxabi/src/CMakeLists.txt

 endif()
 
 include(WarningFlags)
 
 # Build the shared library.
 add_library(cxxabi_shared_objects OBJECT EXCLUDE_FROM_ALL ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
 cxx_add_warning_flags(cxxabi_shared_objects ${LIBCXXABI_ENABLE_WERROR} ${LIBCXXABI_ENABLE_PEDANTIC})
 if (LIBCXXABI_USE_LLVM_UNWINDER)
-  if (LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY)
+  if (LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY OR
+      (DEFINED LIBUNWIND_ENABLE_SHARED AND NOT LIBUNWIND_ENABLE_SHARED))
     target_link_libraries(cxxabi_shared_objects PUBLIC unwind_shared_objects) # propagate usage requirements
     target_sources(cxxabi_shared_objects PUBLIC $<TARGET_OBJECTS:unwind_shared_objects>)
   else()
     target_link_libraries(cxxabi_shared_objects PUBLIC unwind_shared)
   endif()
 endif()
 target_link_libraries(cxxabi_shared_objects PRIVATE cxx-headers ${LIBCXXABI_BUILTINS_LIBRARY} ${LIBCXXABI_SHARED_LIBRARIES} ${LIBCXXABI_LIBRARIES})
 target_link_libraries(cxxabi_shared_objects PUBLIC cxxabi-headers)