[clang] [libcxx] [libcxxabi] [Fuchsia][cmake] Allow using FatLTO when building runtimes (PR #112277)

[Paul Kirth via cfe-commits]

ilovepi wrote:

I'm now getting an error w/ ToT. Not sure why I'm running into it now, but it happens with any clean build, so its deterministic.

**The Relevant bits:**
``` console
ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO
ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO
```

**Full Error Log:**
```console
[1761/1802] Linking CXX shared library /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libc++abi.so.1.0
FAILED: /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libc++abi.so.1.0
: && /usr/local/google/home/paulkirth/llvm-fork/build/./bin/clang++ --target=aarch64-unknown-fuchsia --sysroot=/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot -fPIC --target=aarch64-unknown-fuchsia -I/usr/local/google/home/paulkirth/fuchsia-idk/pkg/sync/include -I/usr/local/google/home/paulkirth/fuchsia-idk/pkg/fdio/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -ffunction-sections -fdata-sections -flto -ffat-lto-objects  -O2 -g -DNDEBUG  -L/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/lib -Wl,-z,defs -fuse-ld=lld -flto -ffat-lto-objects  -nostdlib++ --unwindlib=none -shared -Wl,-soname,libc++abi.so.1 -o /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libc++abi.so.1.0 libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_demangle.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception_storage.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_guard.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_handlers.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_vector.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_virtual.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_exception.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_stdexcept.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_typeinfo.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/abort_message.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/fallback_malloc.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/private_typeinfo.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_new_delete.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.obj libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_thread_atexit.cpp.obj  -lc  /usr/local/google/home/paulkirth/llvm-fork/build/lib/aarch64-unknown-fuchsia/libunwind.so.1.0 && :
ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO
ld.lld: error: input file '/usr/local/google/home/paulkirth/fuchsia-idk/arch/arm64/sysroot/lib/libzircon.so' added after LTO
ld.lld: error: undefined symbol: _zx_system_get_features
>>> referenced by fuchsia.inc:9 (/usr/local/google/home/paulkirth/llvm-fork/compiler-rt/lib/builtins/cpu_model/aarch64/lse_atomics/fuchsia.inc:9)
>>>               aarch64.c.obj:(init_have_lse_atomics) in archive /usr/local/google/home/paulkirth/llvm-fork/build/lib/clang/20/lib/aarch64-unknown-fuchsia/libclang_rt.builtins.a
>>> referenced by fuchsia.inc:12 (/usr/local/google/home/paulkirth/llvm-fork/compiler-rt/lib/builtins/cpu_model/aarch64/fmv/fuchsia.inc:12)
>>>               aarch64.c.obj:(__init_cpu_features_resolver) in archive /usr/local/google/home/paulkirth/llvm-fork/build/lib/clang/20/lib/aarch64-unknown-fuchsia/libclang_rt.builtins.a
clang++: error: ld.lld command failed with exit code 1 (use -v to see invocation)
[1763/1802] Building CXX object libcxx/src/CMakeFiles/cxx_static.dir/locale.cpp.obj
ninja: build stopped: subcommand failed.
```


https://github.com/llvm/llvm-project/pull/112277