[Mac/GN] Add a new linker_driver.py option to save unstripped linker output.

build/config/mac/BUILD.gn

Index: build/config/mac/BUILD.gn
diff --git a/build/config/mac/BUILD.gn b/build/config/mac/BUILD.gn
index d49db4383625bb254a07fcca49377c13ce8f2b55..8a4010c037edc1a9216bdee44c21d5096d04b8fe 100644
--- a/build/config/mac/BUILD.gn
+++ b/build/config/mac/BUILD.gn
@@ -98,6 +98,7 @@ config("mac_executable_flags") {
 
 # The ldflags referenced below are handled by
 # //build/toolchain/mac/linker_driver.py.
+
 # Remove this config if a target wishes to change the arguments passed to the
 # strip command during linking. This config by default strips all symbols
 # from a binary, but some targets may wish to specify a saves file to preserve
@@ -107,3 +108,10 @@ config("strip_all") {
     ldflags = [ "-Wcrl,strip,-x,-S" ]
   }
 }
+
+# This config will keep unstripped copies of the linker output.
+config("save_unstripped_output") {
+  if (save_unstripped_output) {
+    ldflags = [ "-Wcrl,unstripped," + rebase_path(root_out_dir) ]
+  }
+}