[BOLT] Fix invalid abbrev error when reading debug_info section with readelf

Summary:
This fixes a bug which causes the debug_info and debug_loc sections to be unreadable by readelf/objdump.

Basically, we're using 12 bytes of a ULEB128 value to fill in space, but readelf can't read more than 9 bytes of ULEB128. Thus, we replace that value with a string of 'a' instead.

(cherry picked from FBD18097728)