Reland D75382 "[lld] Initial commit for new Mach-O backend"

With a fix for http://lab.llvm.org:8011/builders/clang-cmake-armv8-lld/builds/3636

Also trims some unneeded dependencies.

utils/gn/secondary/lld/MachO/BUILD.gn

@@ -0,0 +1,32 @@
+import("//llvm/utils/TableGen/tablegen.gni")
+
+tablegen("Options") {
+  visibility = [ ":MachO2" ]
+  args = [ "-gen-opt-parser-defs" ]
+}
+
+static_library("MachO2") {
+  output_name = "lldMachO2"
+  configs += [ "//llvm/utils/gn/build:lld_code" ]
+  deps = [
+    ":Options",
+    "//lld/Common",
+    "//llvm/lib/BinaryFormat",
+    "//llvm/lib/Object",
+    "//llvm/lib/Option",
+    "//llvm/lib/Support",
+    "//llvm/lib/Target:TargetsToBuild",
+  ]
+  include_dirs = [ "." ]
+  sources = [
+    "Arch/X86_64.cpp",
+    "Driver.cpp",
+    "InputFiles.cpp",
+    "InputSection.cpp",
+    "OutputSegment.cpp",
+    "SymbolTable.cpp",
+    "Symbols.cpp",
+    "Target.cpp",
+    "Writer.cpp",
+  ]
+}

utils/gn/secondary/lld/tools/lld/BUILD.gn

@@ -27,6 +27,7 @@ executable("lld") {
   deps = [
     "//lld/COFF",
     "//lld/ELF",
+    "//lld/MachO:MachO2",
     "//lld/MinGW",
     "//lld/lib/Driver",
     "//lld/wasm",