[llvm/cmake/config-ix.cmake] If LD64_EXECUTABLE is already set, avoid the need to look up ld64

This provides option to set the `LD64_EXECUTABLE` variable to a path at CMake configure time directly.

(cherry picked from commit 05a165b)

Author: akyrtzi

llvm/cmake/config-ix.cmake

@@ -615,15 +615,17 @@ if(CMAKE_GENERATOR MATCHES "Ninja" AND
 endif()
 
 if(CMAKE_HOST_APPLE AND APPLE)
-  if(NOT CMAKE_XCRUN)
-    find_program(CMAKE_XCRUN NAMES xcrun)
-  endif()
-  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")
+  if(NOT LD64_EXECUTABLE)
+    if(NOT CMAKE_XCRUN)
+      find_program(CMAKE_XCRUN NAMES xcrun)
+    endif()
+    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()
 
   if(LD64_EXECUTABLE)