unix: use a type equivalent to reflect.SliceHeader to manipulate slices

The regression test included in this change verifies that the type is,
in fact, equivalent, while allowing the actual header definitions to
avoid importing the (relatively heavy) "reflect" package itself.

This change is loosely based on Keyan Pishdadian's draft in CL 230557.

For golang/go#37805

Change-Id: I998c69cdeb852154cd66ab5fdaa542a6f19666a2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/231177
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_unix.go

@@ -12,6 +12,8 @@ import (
 	"sync"
 	"syscall"
 	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
 )
 
 var (
@@ -113,15 +115,12 @@ func (m *mmapper) Mmap(fd int, offset int64, length int, prot int, flags int) (d
 		return nil, errno
 	}
 
-	// Slice memory layout
-	var sl = struct {
-		addr uintptr
-		len  int
-		cap  int
-	}{addr, length, length}
-
-	// Use unsafe to turn sl into a []byte.
-	b := *(*[]byte)(unsafe.Pointer(&sl))
+	// 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
 
 	// Register mapping in m and return it.
 	p := &b[cap(b)-1]