write: need better handling of DW_FORM_ref_addr

[philipc]

The current handling of DW_FORM_ref_addr assumes that you are writing all of the compilation units at once, so that it can calculate the section offset itself, but that is atypical behaviour. Instead, the producer will normally need to generate a symbol and relocation so that the reference can be resolved during static linking.

Fixing this will need a means to assign a symbol to the reference, and a means to assign a symbol to an entry that may be referred to.

This will also apply to DW_OP_call_ref in expressions.