Update lib/ section names
Update Darwin debug section names
Use short names for section begin/end labels
Use private label prefix and indirect info length in debug sections
Correct debug info length
Add DW_AT_comp_dir to dwarf compile unit

This allows debuggers to find nested source files
Don't reference .debug_line via DW_AT_stmt_list on Darwin
Run dsymutil if generating debuggable code

(Darwin only)
Rename ucc's tempfile ASSIGN() -> FILL_WITH_TMP()
Only dsymutil if temporary object files were used