wasip1 syscall package needs syscall.FDFLAG_NONBLOCK

[dgryski]

Go's internal/syscall/unix package expects it, and is needed for some tests

~ $ tinygo test -target=wasip1 mime/multipart
FAIL	mime/multipart	0.000s
# internal/syscall/unix
/Users/dgryski/go/src/go.googlesource.com/go/src/internal/syscall/unix/nonblocking_wasip1.go:19:23: undefined: syscall.FDFLAG_NONBLOCK
/Users/dgryski/go/src/go.googlesource.com/go/src/internal/syscall/unix/nonblocking_wasip1.go:23:22: undefined: syscall.FDFLAG_NONBLOCK
~ $ tinygo test -target=wasip1 net/mail
FAIL	net/mail	0.000s
# internal/syscall/unix
/Users/dgryski/go/src/go.googlesource.com/go/src/internal/syscall/unix/nonblocking_wasip1.go:19:23: undefined: syscall.FDFLAG_NONBLOCK
/Users/dgryski/go/src/go.googlesource.com/go/src/internal/syscall/unix/nonblocking_wasip1.go:23:22: undefined: syscall.FDFLAG_NONBLOCK
~ $ tinygo test -target=wasip1 net/textproto
FAIL	net/textproto	0.000s
# internal/syscall/unix
/Users/dgryski/go/src/go.googlesource.com/go/src/internal/syscall/unix/nonblocking_wasip1.go:19:23: undefined: syscall.FDFLAG_NONBLOCK
/Users/dgryski/go/src/go.googlesource.com/go/src/internal/syscall/unix/nonblocking_wasip1.go:23:22: undefined: syscall.FDFLAG_NONBLOCK

[aykevl]

I was actually changing some targets over to use the Go stdlib syscall package instead. That would probably solve this issue.
See: #4461 for example.