cmd/compile: use (*strings.Reader).Reset

cmd/compile: use (*strings.Reader).Reset

Since when go1.17 is now used for bootstraping.

Change-Id: If28338fc82e6c61f057d7eb3a4c2ed77846167df
Reviewed-on: https://go-review.googlesource.com/c/go/+/427558
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>