[lld] [lld-macho] Link against libatomic when necessary (PR #144259)

[via llvm-commits]

https://github.com/sertonix created https://github.com/llvm/llvm-project/pull/144259

In Driver.cpp std::atomic<uint64_t> is used which may need libatomic.

Build failure (if that is of interest):
```
[127/135] Linking CXX shared library lib/liblldMachO.so.20.1
ninja: job failed: : && /usr/lib/ccache/bin/clang++-20 -fPIC -Os -fstack-clash-protection -Wformat -Werror=format-security -D_GLIBCXX_ASSERTIONS=1 -D_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS=1 -D_LIBCPP_ENABLE_HARDENED_MODE=1 -g -O2 -DNDEBUG -g1 -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 -fdiagnostics-color -ffunction-sections -fdata-sections  -Wl,--as-needed,-O1,--sort-common -Wl,-z,defs -Wl,-z,nodelete   -Wl,-rpath-link,/home/user/aports/main/lld20/src/lld-20.1.5.src/build/./lib  -Wl,--gc-sections -shared -Wl,-soname,liblldMachO.so.20.1 -o lib/liblldMachO.so.20.1 MachO/CMakeFiles/lldMachO.dir/Arch/ARM64.cpp.o MachO/CMakeFiles/lldMachO.dir/Arch/ARM64Common.cpp.o MachO/CMakeFiles/lldMachO.dir/Arch/ARM64_32.cpp.o MachO/CMakeFiles/lldMachO.dir/Arch/X86_64.cpp.o MachO/CMakeFiles/lldMachO.dir/ConcatOutputSection.cpp.o MachO/CMakeFiles/lldMachO.dir/Driver.cpp.o MachO/CMakeFiles/lldMachO.dir/DriverUtils.cpp.o MachO/CMakeFiles/lldMachO.dir/Dwarf.cpp.o MachO/CMakeFiles/lldMachO.dir/EhFrame.cpp.o MachO/CMakeFiles/lldMachO.dir/ExportTrie.cpp.o MachO/CMakeFiles/lldMachO.dir/ICF.cpp.o MachO/CMakeFiles/lldMachO.dir/InputFiles.cpp.o MachO/CMakeFiles/lldMachO.dir/InputSection.cpp.o MachO/CMakeFiles/lldMachO.dir/LTO.cpp.o MachO/CMakeFiles/lldMachO.dir/MapFile.cpp.o MachO/CMakeFiles/lldMachO.dir/MarkLive.cpp.o MachO/CMakeFiles/lldMachO.dir/ObjC.cpp.o MachO/CMakeFiles/lldMachO.dir/OutputSection.cpp.o MachO/CMakeFiles/lldMachO.dir/OutputSegment.cpp.o MachO/CMakeFiles/lldMachO.dir/Relocations.cpp.o MachO/CMakeFiles/lldMachO.dir/BPSectionOrderer.cpp.o MachO/CMakeFiles/lldMachO.dir/SectionPriorities.cpp.o MachO/CMakeFiles/lldMachO.dir/Sections.cpp.o MachO/CMakeFiles/lldMachO.dir/SymbolTable.cpp.o MachO/CMakeFiles/lldMachO.dir/Symbols.cpp.o MachO/CMakeFiles/lldMachO.dir/SyntheticSections.cpp.o MachO/CMakeFiles/lldMachO.dir/Target.cpp.o MachO/CMakeFiles/lldMachO.dir/UnwindInfoSection.cpp.o MachO/CMakeFiles/lldMachO.dir/Writer.cpp.o -L/usr/lib/llvm20/lib -Wl,-rpath,"\$ORIGIN/../lib:/usr/lib/llvm20/lib:/home/user/aports/main/lld20/src/lld-20.1.5.src/build/lib:"  lib/liblldCommon.so.20.1  /usr/lib/llvm20/lib/libLLVM.so.20.1 && :
/usr/lib/gcc/powerpc-alpine-linux-musl/14.3.0/../../../../powerpc-alpine-linux-musl/bin/ld: MachO/CMakeFiles/lldMachO.dir/Driver.cpp.o: in function `handleExplicitExports()':
/usr/lib/gcc/powerpc-alpine-linux-musl/14.3.0/../../../../include/c++/14.3.0/bits/atomic_base.h:501:(.text._ZL21handleExplicitExportsv+0xb8): undefined reference to `__atomic_load_8'
/usr/lib/gcc/powerpc-alpine-linux-musl/14.3.0/../../../../powerpc-alpine-linux-musl/bin/ld: /usr/lib/gcc/powerpc-alpine-linux-musl/14.3.0/../../../../include/c++/14.3.0/bits/atomic_base.h:501:(.text._ZL21handleExplicitExportsv+0x180): undefined reference to `__atomic_load_8'
/usr/lib/gcc/powerpc-alpine-linux-musl/14.3.0/../../../../powerpc-alpine-linux-musl/bin/ld: MachO/CMakeFiles/lldMachO.dir/Driver.cpp.o: in function `void llvm::function_ref<void (unsigned int)>::callback_fn<llvm::parallelForEach<lld::macho::Symbol* const*, handleExplicitExports()::$_0>(lld::macho::Symbol* const*, lld::macho::Symbol* const*, handleExplicitExports()::$_0)::{lambda(unsigned int)#1}>(int, unsigned int)':
/usr/lib/gcc/powerpc-alpine-linux-musl/14.3.0/../../../../include/c++/14.3.0/bits/atomic_base.h:631:(.text._ZN4llvm12function_refIFvjEE11callback_fnIZNS_15parallelForEachIPKPN3lld5macho6SymbolEZL21handleExplicitExportsvE3$_0EEvT_SC_T0_EUljE_EEvij+0xd4): undefined reference to `__atomic_fetch_add_8'
clang++-20: error: linker command failed with exit code 1 (use -v to see invocation)
```

CC @int3 @gkmhub @smeenai

>From 8c8868f2d208b27113c2c259e85791031604e943 Mon Sep 17 00:00:00 2001
From: Sertonix <sertonix at posteo.net>
Date: Sun, 15 Jun 2025 12:29:57 +0200
Subject: [PATCH] [lld-macho] Link against libatomic when necessary

In Driver.cpp std::atomic<uint64_t> is used which may need libatomic.
---
 lld/MachO/CMakeLists.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lld/MachO/CMakeLists.txt b/lld/MachO/CMakeLists.txt
index ecf6ce609e59f..433ac4ec85853 100644
--- a/lld/MachO/CMakeLists.txt
+++ b/lld/MachO/CMakeLists.txt
@@ -58,6 +58,7 @@ add_lld_library(lldMachO
   LINK_LIBS
   lldCommon
   ${LLVM_PTHREAD_LIB}
+  ${LLVM_ATOMIC_LIB}
 
   DEPENDS
   MachOOptionsTableGen