Removes 'arch' from the cflags field in generated gyp file.

Signed-off-by: Olli Wang <olliwang@ollix.com>
Removes unused target_default_keywords class variable from the Target class.

The class variable is no use anymore after dropping the support of merging common keywords into the "target_defaults" fields from targets.

Signed-off-by: Olli Wang <olliwang@ollix.com>