NetBSD: mu.enter/mu.leave undefined

1.39.0 is broken on NetBSD, breaking packages using the module. I checked git head (ba0b20fc) and it's broken the same way:

# go build
go: downloading golang.org/x/sys v0.37.0
go: downloading modernc.org/libc v1.67.6
go: downloading github.com/mattn/go-isatty v0.0.20
go: downloading modernc.org/memory v1.11.0
go: downloading github.com/dustin/go-humanize v1.0.1
go: downloading github.com/google/uuid v1.6.0
go: downloading golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546
go: downloading modernc.org/mathutil v1.7.1
go: downloading github.com/ncruces/go-strftime v1.0.0
go: downloading github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec
# modernc.org/sqlite/lib
lib/sqlite_netbsd_amd64.go:112447:5: mu.enter undefined (type mutex has no field or method enter)
lib/sqlite_netbsd_amd64.go:112448:11: mu.leave undefined (type mutex has no field or method leave)