[BOLT][DWARF][NFC] Use only skeleton/main CUs to update .debug_aranges

Summary:
Previously, we were registering all CUs with aranges writer. Since DWO
CUs have offsets set to 0, and we were registering them after the
skeleton unit at offset 0 was already registered, it was mostly
harmless as DWO CUs were effectively ignored.

(cherry picked from FBD31162621)