We previously left the amd64_addr_t attribute uninitialized for OP_REG,
OP_REG_REG and OP_REG_IMM. Make the code more consistent by always
initializing it to the newly introduced X86_ADDR_REG variant, as a bonus
we can also use base_input member to indicate the register input instead
of hardcoding it to 0.

This is done by introducing a new new_global_entity() function for
constructing globals/adding a visibility argument to new_alias_entity().

This flag can be set on entities where the actual address isn't
known/compared against other address. This also fixes a bug where we
used the mach-o csrings section for global char arrays which need
distinct addresses.

Default initialisation just happens to set the correct value.