[lld] Initial commit for new Mach-O backend

Summary:
This is the first commit for the new Mach-O backend, designed to roughly
follow the architecture of the existing ELF and COFF backends, and
building off work that @ruiu and @pcc did in a branch a while back. Note
that this is a very stripped-down commit with the bare minimum of
functionality for ease of review. We'll be following up with more diffs
soon.

Currently, we're able to generate a simple "Hello World!" executable
that runs on OS X Catalina (and possibly on earlier OS X versions; I
haven't tested them). (This executable can be obtained by compiling
`test/MachO/relocations.s`.) We're mocking out a few load commands to
achieve this -- for example, we can't load dynamic libraries, but
Catalina requires binaries to be linked against `dyld`, so we hardcode
the emission of a `LC_LOAD_DYLIB` command. Other mocked out load
commands include LC_SYMTAB and LC_DYSYMTAB.

Differential Revision: https://reviews.llvm.org/D75382

2 files changed, 9 insertions, 4 deletions

diff --git a/lld/tools/lld/CMakeLists.txt b/lld/tools/lld/CMakeLists.txt
index a37c2c702bd5..2cdd9b854246 100644
--- a/lld/tools/lld/CMakeLists.txt
+++ b/lld/tools/lld/CMakeLists.txt
@@ -16,6 +16,7 @@ target_link_libraries(lld
   lldCOFF
   lldDriver
   lldELF
+  lldMachO2
   lldMinGW
   lldWasm
   )
diff --git a/lld/tools/lld/lld.cpp b/lld/tools/lld/lld.cpp
index 646fc3d6468e..58a291d1e921 100644
--- a/lld/tools/lld/lld.cpp
+++ b/lld/tools/lld/lld.cpp
@@ -45,10 +45,11 @@ using namespace llvm::sys;
 
 enum Flavor {
   Invalid,
-  Gnu,     // -flavor gnu
-  WinLink, // -flavor link
-  Darwin,  // -flavor darwin
-  Wasm,    // -flavor wasm
+  Gnu,       // -flavor gnu
+  WinLink,   // -flavor link
+  Darwin,    // -flavor darwin
+  DarwinNew, // -flavor darwinnew
+  Wasm,      // -flavor wasm
 };
 
 LLVM_ATTRIBUTE_NORETURN static void die(const Twine &s) {
@@ -62,6 +63,7 @@ static Flavor getFlavor(StringRef s) {
       .CasesLower("wasm", "ld-wasm", Wasm)
       .CaseLower("link", WinLink)
       .CasesLower("ld64", "ld64.lld", "darwin", Darwin)
+      .CaseLower("darwinnew", DarwinNew)
       .Default(Invalid);
 }
 
@@ -157,6 +159,8 @@ int main(int argc, const char **argv) {
     return !coff::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case Darwin:
     return !mach_o::link(args, canExitEarly(), llvm::outs(), llvm::errs());
+  case DarwinNew:
+    return !macho::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case Wasm:
     return !wasm::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   default: