Default-enable external startup data for Linux for stand-alone builds.

Notes:
- Other platforms to follow later.
- This follows Chromium practice, that mostly uses this feature these days.
- The statically linked-in startup data will stay. So whoever prefers
  the old way just needs to set the flag differently.

Reland crrev.com/959693002, once crrev.com/960883003 is in.

R=machenbach@chromium.org
BUG=

Review URL: https://codereview.chromium.org/956373002

Cr-Commit-Position: refs/heads/master@{#26980}

build/standalone.gypi

@@ -152,6 +152,11 @@
       }, {
         'clang%': 0,
       }],
+      ['(OS=="linux")', {
+        # Gradually roll out v8_use_external_startup_data.
+        # Should eventually be default enabled on all platforms.
+        'v8_use_external_startup_data%': 1,
+      }],
     ],
     # Default ARM variable settings.
     'arm_version%': 'default',