Re-land: Enable external startup by default on Linux.

crrev.com/1016603004 should have fixed the issues that broke this last time.

Notes:
- The current functionality is meant to stay. If you prefer the old way,
  just set v8_use_external_startup_data=0.
- You might notice this if you call d8 via PATH. Either explicitly tell d8
  where your external snapshot/built-ins are, or change the build options
  as above.
- This follows Chromium practice, that mostly uses this feature these days.
- Other platforms to follow later.

BUG=

Review URL: https://codereview.chromium.org/1041683002

Cr-Commit-Position: refs/heads/master@{#28858}

build/standalone.gypi

@@ -201,6 +201,11 @@
         # the C++ standard library is used.
         'use_custom_libcxx%': 1,
       }],
+      ['OS=="linux" and v8_target_arch==host_arch', {
+        # 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',