[build] Enable ICF for Mac.

xcodebuild/ReleaseARM64/exe.stripped/dart -131k (-3.25%)

Change-Id: I9d3f4f39c77bb242e891e1996e217923bbc55d0f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/482984
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

build/config/compiler/BUILD.gn

@@ -887,12 +887,6 @@ if (is_win) {
       "-Wl,--gc-sections",
     ]
 
-    if (is_clang && !using_sanitizer) {
-      # Identical code folding to reduce size.
-      # Warning: This changes C/C++ semantics of function pointer comparison.
-      common_optimize_on_ldflags += [ "-Wl,--icf=all" ]
-    }
-
     if (!using_sanitizer) {
       # Functions interposed by the sanitizers can make ld think
       # that some libraries aren't needed when they actually are,
@@ -900,6 +894,13 @@ if (is_win) {
       common_optimize_on_ldflags += [ "-Wl,--as-needed" ]
     }
   }
+
+  if (is_clang && !using_sanitizer) {
+    # Identical code folding to reduce size.
+    # Warning: This changes C/C++ semantics of function pointer comparison.
+    # Needs lld/gold/mold instead of GNU ld/Mac ld.
+    common_optimize_on_ldflags += [ "-Wl,--icf=all" ]
+  }
 }
 
 # Default "optimization on" config. On Windows, this favors size over speed.