Added conversion from immediate value to effective address. Used for enter

Added conversion from immediate value to effective address.  Used for enter
instruction (which takes two immediate values).
To facilitate this change, changed meaning of "len" in bytecode structure
to be only the length of the offset (from being 1+length, the 1 being for the
Mod/RM byte), and added a need_modrm flag similar to the need_sib flag in
functionality.

svn path=/trunk/yasm/; revision=22