This extends 9f1f04f313 to the other
implementations.
|EC_GFp_nistp224_method| and |EC_GFp_nistp256_method| are not marked
|OPENSSL_EXPORT|. |EC_GROUP_set_generator| doesn't allow the generator
to be changed for any |EC_GROUP| for built-in curves. Consequently,
there's no way (except some kind of terrible abuse) that this code
could be executed with a non-default generator.
Change-Id: I5d9b6be4e6f9d384159cb3d708390a8e3c69f23f
Reviewed-on: https://boringssl-review.googlesource.com/6489
Reviewed-by: Adam Langley <agl@google.com>