[codegen] add to_static_str method to IntCC
[codegen] add to_static_str method to IntCC

Add a method to_static_str to objects of type IntCC which consumes the
object to basically do the opposite of IntCC::new.

Refs: https://github.com/CraneStation/cranelift/pull/1081#discussion_r329042331

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne