feat: Support cross compiles from linux to macos

Upstreaming an internal patch we've been using
for cross compiling the aspect CLI from a linux
machine to mac. We've been using a slightly
different version of this for quite some time
now so I cleaned up the logic significatly so it
could be upstreamed.

Author: mrmeku

toolchain/cc_toolchain_config.bzl

@@ -168,11 +168,17 @@ def cc_toolchain_config(
     archive_flags = []
 
     # Linker flags:
-    if exec_os == "darwin" and not is_xcompile:
-        # lld is experimental for Mach-O, so we use the native ld64 linker.
-        # TODO: How do we cross-compile from Linux to Darwin?
-        use_lld = False
+    ld = "ld.lld"
+    if target_os == "darwin":
+        use_lld = True
+
+        ld = "ld64.lld"
+        ld_path = toolchain_path_prefix + "/bin/" + ld
+        compile_flags.append("-mmacosx-version-min=12.0")
         link_flags.extend([
+            "-mmacosx-version-min=12.0",
+            "-Wl,-platform_version,macos,12.0,12.0",
+            "--ld-path=" + ld_path,
             "-headerpad_max_install_names",
             "-fobjc-link-runtime",
         ])
@@ -195,7 +201,9 @@ def cc_toolchain_config(
         # not an option because it is not a cross-linker, so lld is the
         # only option.
         use_lld = True
+        ld_path = toolchain_path_prefix + "/bin/" + ld
         link_flags.extend([
+            "--ld-path=" + ld_path,
             "-fuse-ld=lld",
             "-Wl,--build-id=md5",
             "-Wl,--hash-style=gnu",
@@ -323,7 +331,7 @@ def cc_toolchain_config(
         "dwp": tools_path_prefix + "llvm-dwp",
         "gcc": wrapper_bin_prefix + "cc_wrapper.sh",
         "gcov": tools_path_prefix + "llvm-profdata",
-        "ld": tools_path_prefix + "ld.lld" if use_lld else "/usr/bin/ld",
+        "ld": tools_path_prefix + ld if use_lld else "/usr/bin/ld",
         "llvm-cov": tools_path_prefix + "llvm-cov",
         "llvm-profdata": tools_path_prefix + "llvm-profdata",
         "nm": tools_path_prefix + "llvm-nm",

toolchain/internal/common.bzl

@@ -28,6 +28,7 @@ _toolchain_tools = {
     for name in [
         "clang-cpp",
         "ld.lld",
+        "ld64.lld",
         "llvm-ar",
         "llvm-dwp",
         "llvm-profdata",