syscall: simplify unsafe.Slice usage

syscall: simplify unsafe.Slice usage

Same as CL 429915.

Change-Id: I83f4d3bd980294d5bae387d875368b069be2d91a
Reviewed-on: https://go-review.googlesource.com/c/go/+/429955
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>