Make self-linked shared objects dlopen/Fiddle-compatible

[katsyoshi]

Goal

Make self-linked shared objects (shared: true) usable from dlopen/Fiddle.

Acceptance Criteria

• nm -D libadd.so shows exported symbols (e.g. add).
• readelf -d libadd.so has consistent DT_SYMTAB/DT_STRTAB/DT_STRSZ/DT_HASH.
• RUBY_BOX=1 ruby test/caotral/linker/fiddle_test.rb passes.

Tasks

• Populate .dynsym with defined non-local symbols for shared objects.
• Append symbol names to .dynstr and wire st_name offsets.
• Set .dynsym symbol fields (st_value, st_shndx, st_size, st_info) for exported entries.
• Emit .hash for shared objects (not only PIE) and patch DT_HASH.
• Ensure writer patches DT_SYMTAB/DT_STRTAB/DT_STRSZ/DT_SYMENT from final layout.
• Add/maintain a minimal Fiddle smoke test (add.c) with cleanup in teardown.
• Fix test input generation to use object files (gcc -fPIC -c) instead of -shared -o *.o.