all: simplify unsafe.Slice usage in {Byte,UTF16}PtrToString

On windows, use unsafe.Slice instead of unsafeheader as already the case
for unix and plan9.

The pointers are already *byte/*uint16, so the type conversion can be
omitted as well.

Change-Id: Ida7264cc0c1948bf563ed91d51e637edcdafb77a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/430515
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

unix/syscall.go

@@ -80,8 +80,7 @@ func BytePtrToString(p *byte) string {
 		ptr = unsafe.Pointer(uintptr(ptr) + 1)
 	}
 
-	s := unsafe.Slice((*byte)(unsafe.Pointer(p)), n)
-	return string(s)
+	return string(unsafe.Slice(p, n))
 }
 
 // Single-word zero for use when we need a valid pointer to 0 bytes.