platform2: move --std=gnu++11 flag to common-mk/common.gypi

A number of platform2 projects enabled C++11 features individually
in their .GYP files. Removed this flag from each of the projects
and added it to common.gypi for everyone to share.

BUG=None
TEST=Compile various platform2 targets to make sure they still compile.

Change-Id: I4136573ec5dcc988df2826056c8dae8847725504
Reviewed-on: https://chromium-review.googlesource.com/211534
Reviewed-by: Alex Vakulenko <[email protected]>
Commit-Queue: Alex Vakulenko <[email protected]>
Tested-by: Alex Vakulenko <[email protected]>
diff --git a/attestation.gyp b/attestation.gyp
index 728761f..2833b90 100644
--- a/attestation.gyp
+++ b/attestation.gyp
@@ -11,7 +11,6 @@
         'protobuf',
       ]
     },
-    'cflags_cc': [ '-std=gnu++11' ],
   },
   'targets': [
     {