[cmake,Apple] Check ld-classic when looking for ld64. (llvm#77136)

In newer SDKs, ld64 is called ld-classic. Look for it first, as ld in
those SDKs is still missing some functionality some tests depend on.

This enables running the tests from check-llvm-tools-lto with newer SDKs
on ARM64 macOS.

Author: fhahn

llvm/cmake/config-ix.cmake

@@ -639,12 +639,25 @@ if(CMAKE_HOST_APPLE AND APPLE)
     if(NOT CMAKE_XCRUN)
       find_program(CMAKE_XCRUN NAMES xcrun)
     endif()
+
+    # First, check if there's ld-classic, which is ld64 in newer SDKs.
     if(CMAKE_XCRUN)
-      execute_process(COMMAND ${CMAKE_XCRUN} -find ld
+      execute_process(COMMAND ${CMAKE_XCRUN} -find ld-classic
         OUTPUT_VARIABLE LD64_EXECUTABLE
         OUTPUT_STRIP_TRAILING_WHITESPACE)
     else()
-      find_program(LD64_EXECUTABLE NAMES ld DOC "The ld64 linker")
+      find_program(LD64_EXECUTABLE NAMES ld-classic DOC "The ld64 linker")
+    endif()
+
+    # Otherwise look for ld directly.
+    if(NOT LD64_EXECUTABLE)
+        if(CMAKE_XCRUN)
+          execute_process(COMMAND ${CMAKE_XCRUN} -find ld
+            OUTPUT_VARIABLE LD64_EXECUTABLE
+            OUTPUT_STRIP_TRAILING_WHITESPACE)
+        else()
+          find_program(LD64_EXECUTABLE NAMES ld DOC "The ld64 linker")
+        endif()
     endif()
   endif()