modernc.org/libc v1.73.1 Fix netbsd/amd64 Xmmap for the NetBSD mmap(2) PAD ABI: the offset must be passed as the 7th syscall argument (Syscall9), not the 6th (Syscall6), or it lands in the `long PAD` slot and off_t pos is left as stack garbage, returning an unaligned / unbacked mapping that faults on first access (SIGBUS in the SQLite WAL-index shared memory under concurrent WAL). Only libc_netbsd.go changes; identical to v1.73.0 for all other targets. Resolves modernc.org/sqlite#246 on netbsd/amd64.