Add type hints to mk2rbc

Type hints have the format #RBC# type_hint MY_VAR list
and must be specified at the top of the Makefile. Setting
one will cause that variable to have that type for the
remainder of the Makefile. This can be used where mk2rbc's
type inference detects the wrong type and it must be
manually changed.

Bug: 224601891
Test: go test
Change-Id: I6db2c50056d0298227e1d2801a522adf8bbd1df8