[CMake][AIX] Add `-bcdtors:mbr` option when building with IBM XL

[hubert.reinterpretcast]

Summary

The -bcdtors:mbr option causes processing for constructors and destructors to omit otherwise-unreferenced members of static libraries, matching the processing done on Linux, where --whole-archive is not the default. Applying this option is desirable for reducing the footprint of an installation.

[daltenty]

llvm/cmake/modules/HandleLLVMOptions.cmake

202

This seems more generally applicable on AIX. Could we promote this out of the XL specific block and just pass it as a general linker option?

[hubert.reinterpretcast]

llvm/cmake/modules/HandleLLVMOptions.cmake

202

I don't think we should move this out of the XL-specific block at this time. GCC does not use the same mechanism for static initialization on AIX.

[daltenty]

llvm/cmake/modules/HandleLLVMOptions.cmake

200

--whole-archive is used on Linux for building the shared libraries when -DLLVM_BUILD_LLVM_DYLIB=yes. Do we need to be concerned about this case?

[daltenty]

LGTM

llvm/cmake/modules/HandleLLVMOptions.cmake

200

Summary of offline discussion: AIX would use the export list mechanism in place of --whole-archive, and the export list would make those symbols referenced so we’d pull in the initialization even with -bcdtors:mbr

llvm/cmake/modules/HandleLLVMOptions.cmake

 if(${CMAKE_SYSTEM_NAME} MATCHES "AIX")
   # -fPIC does not enable the large code model for GCC on AIX but does for XL.
   if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     append("-mcmodel=large" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
   elseif(CMAKE_CXX_COMPILER_ID MATCHES "XL")
     # XL generates a small number of relocations not of the large model, -bbigtoc is needed.
     append("-Wl,-bbigtoc"
            CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+    # The default behaviour on AIX processes dynamic initialization of non-local variables with
+    # static storage duration even for archive members that are otherwise unreferenced.
+    # Since `--whole-archive` is not used by the LLVM build to keep such initializations for Linux,
+    # we can limit the processing for archive members to only those that are otherwise referenced.
+    append("-bcdtors:mbr"
+           CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   endif()
 endif()
 
 # Pass -Wl,-z,defs. This makes sure all symbols are defined. Otherwise a DSO
 # build might work on ELF but fail on MachO/COFF.
 if(NOT (${CMAKE_SYSTEM_NAME} MATCHES "Darwin|FreeBSD|OpenBSD|DragonFly|AIX|SunOS" OR
         WIN32 OR CYGWIN) AND
    NOT LLVM_USE_SANITIZER)