mac: Pass -no_deduplicate to mac linker for v8_context_snapshot_generator

We disable ICF for v8_context_snapshot_generator on Linux and Windows.
Now we also do it on macOS.

(I noticed that we weren't doing it because the mac/lld build complained
about ld64.lld not understanding --icf=none.)

Bug: 617892,1149587
Change-Id: Idf40a021ec70dea394ed7b74826ceaed99d853b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591007
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836837}

tools/v8_context_snapshot/BUILD.gn

@@ -87,8 +87,9 @@ if (use_v8_context_snapshot) {
     outputs = [ output_file ]
   }
 
-  # This config disables a link time optimization "ICF", which may merge different
-  # functions into one if the function signature and body of them are identical.
+  # This config disables a link time optimization "ICF", which may merge
+  # different functions into one if the function signature and body of them are
+  # identical.
   #
   # ICF breaks 1:1 mappings of the external references for V8 snapshot, so we
   # disable it while taking a V8 snapshot.
@@ -96,6 +97,8 @@ if (use_v8_context_snapshot) {
     visibility = [ ":*" ]  # Only targets in this file can depend on this.
     if (is_win) {
       ldflags = [ "/OPT:NOICF" ]  # link.exe, but also lld-link.exe.
+    } else if (is_apple) {
+      ldflags = [ "-Wl,-no_deduplicate" ]  # ld64, and ld64.lld.
     } else if (use_gold || use_lld) {
       ldflags = [ "-Wl,--icf=none" ]
     }