unix: use unsafe.Slice instead of unsafeheader package

Go 1.18 is the minimum supported Go version and unsafe.Slice was
introduced in Go 1.17.

Change-Id: Ie5f1fad01f219e7b7a190de2c49676f366ad3bc7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/428515
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_unix.go

@@ -13,8 +13,6 @@ import (
 	"sync"
 	"syscall"
 	"unsafe"
-
-	"golang.org/x/sys/internal/unsafeheader"
 )
 
 var (
@@ -117,11 +115,7 @@ func (m *mmapper) Mmap(fd int, offset int64, length int, prot int, flags int) (d
 	}
 
 	// Use unsafe to convert addr into a []byte.
-	var b []byte
-	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&b))
-	hdr.Data = unsafe.Pointer(addr)
-	hdr.Cap = length
-	hdr.Len = length
+	b := unsafe.Slice((*byte)(unsafe.Pointer(addr)), length)
 
 	// Register mapping in m and return it.
 	p := &b[cap(b)-1]