Fails to build in gccgo

[JeremyRand]

$ go build -v
github.com/phuslu/log
# github.com/phuslu/log
./async_linux.go:24:29: error: incompatible types in assignment (cannot use type uint64 as type syscall.Iovec_len_t)
   24 |                 iovs[0].Len = uint64(len(es[0].buf))
      |                             ^
./async_linux.go:38:37: error: incompatible types in assignment (cannot use type uint64 as type syscall.Iovec_len_t)
   38 |                         iovs[n].Len = uint64(len(es[n].buf))

Go version is:

go version go1.18 gccgo (Debian 14.2.0-19) 14.2.0 linux/ppc64

I'm not sure how easy it is to fix, might be as simple as adding a cast. Would you accept a PR for this, if I can get it fixed? (Or if you'd like to fix it yourself, that'd certainly work for me too.)

[phuslu]

Sorry I have no gccgo environment, PR is welcome. thanks in advance.