ELF builder sets the Thumb interworking bit on STT_FUNC symbols for A32 objects #598

Found by the #596 lane: st_value |= 1 (Thumb bit) is applied to STT_FUNC symbols even when the object is A32 (cortex-r5 path) — harnesses must mask bit 0 as a workaround (documented in call_indirect_594_differential.py). Fix: only set the bit for Thumb-encoded functions. Low severity (linkers tolerate it today) but wrong metadata; same family as the #489 symbol-hygiene items.