internal/syscall/unix: add hurd build tag
internal/syscall/unix: add hurd build tag

Patch from Svante Signell.

Fixes GCC PR go/93900

Change-Id: I3a86102858d13d16ddec0c646867aed2d4cb6c78
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/220589
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend