Add flags attribute to go_cross_binary

[udf2457]

It would be great if flags could be specified in go_cross_binary()

For example, a common flagset when compiling Go is -s -w -buildmode=pie.

However certain platforms (e.g. BSD) are incompatibble with -buildmode=pie.

Therefore when using a cross-compile mechanism such as go_cross_binary() it is naturally important to be able to specify flag differences.