Hojun-Cho
1b0955c97b
First step toward top-level mutable `let`. Adds a sibling directive to DATA whose bytes land in a separate writable .data PROGBITS section (SHF_ALLOC|SHF_WRITE, STT_OBJECT) instead of .text. The section is emitted only when DATAW was used, so inputs without it produce a byte-identical .o — tests 991 (selfhost .o diff) and 995 (self-rebuild) keep passing unchanged. w6l still treats data-resident syms as undefined; that's the next step.