Link liblldCOFF against libatomic when necessary

Also simplify code for liblldCommon using the new LLVM_ATOMIC_LIB variable.

Depends on D136280.

Bug: https://bugs.gentoo.org/832675
Thanks-to: Arfrever Frehtes Taifersar Arahesis <Arfrever@Apache.Org>
Tested-by: erhard_f@mailbox.org <erhard_f@mailbox.org>

Differential Revision: https://reviews.llvm.org/D136281

(cherry picked from commit f0b451c)

Author: thesamesam

lld/COFF/CMakeLists.txt

@@ -44,6 +44,7 @@ add_lld_library(lldCOFF
   LINK_LIBS
   lldCommon
   ${LLVM_PTHREAD_LIB}
+  ${LLVM_ATOMIC_LIB}
 
   DEPENDS
   COFFOptionsTableGen

lld/Common/CMakeLists.txt

@@ -1,9 +1,3 @@
-set(LLD_SYSTEM_LIBS ${LLVM_PTHREAD_LIB})
-
-if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
-  list(APPEND LLD_SYSTEM_LIBS atomic)
-endif()
-
 find_first_existing_vc_file("${LLVM_MAIN_SRC_DIR}" llvm_vc)
 find_first_existing_vc_file("${LLD_SOURCE_DIR}" lld_vc)
 
@@ -54,7 +48,8 @@ add_lld_library(lldCommon
   Target
 
   LINK_LIBS
-  ${LLD_SYSTEM_LIBS}
+  ${LLVM_PTHREAD_LIB}
+  ${LLVM_ATOMIC_LIB}
 
   DEPENDS
   intrinsics_gen