Handle signed values in parseHexField by falling back to int64_t parsing
when uint64_t fails. This allows pre-aggregated profile tools to use -1
for BR_ONLY, -2 for FT_EXTERNAL_ORIGIN, -3 for FT_EXTERNAL_RETURN.
Guard the external address reset loop in parseAggregatedLBREntry to
preserve sentinel values (offsets >= FT_EXTERNAL_RETURN).
Add tests for -1/-2/-3 in parseHexField and T entries with -1,
ffffffffffffffff, and buildid:-1 as BR_ONLY.
46154fef0e