[gn build] Port fe3406e

Author: llvmgnsyncbot

llvm/utils/gn/secondary/lld/ELF/BUILD.gn

@@ -39,6 +39,7 @@ static_library("ELF") {
     "Arch/PPC64.cpp",
     "Arch/RISCV.cpp",
     "Arch/SPARCV9.cpp",
+    "Arch/SystemZ.cpp",
     "Arch/X86.cpp",
     "Arch/X86_64.cpp",
     "CallGraphSort.cpp",