[dsymutil] Explicitly link against libatomic when necessary

In some systems, such as RISC-V, atomic support requires explicit linking
against '-latomic' (see riscvarchive/riscv-gcc#12).

Reviewers: davezarzycki, hhb, beanz, jfb, JDevlieghere
Reviewed By: beanz, JDevlieghere
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69003

llvm/tools/dsymutil/CMakeLists.txt

@@ -37,3 +37,7 @@ add_llvm_tool(dsymutil
 if(APPLE)
   target_link_libraries(dsymutil PRIVATE "-framework CoreFoundation")
 endif(APPLE)
+
+if(HAVE_CXX_ATOMICS_WITH_LIB OR HAVE_CXX_ATOMICS64_WITH_LIB)
+  target_link_libraries(dsymutil PRIVATE atomic)
+endif()