runtime: quote -ex and -iex gdb arguments on Windows

runtime: quote -ex and -iex gdb arguments on Windows

On Windows, some gdb flavors expect -ex and -iex arguments containing
spaces to be double quoted.

Change-Id: I2891e115f98c1df3a7a481bd9f9d9215bfbecd44
Reviewed-on: https://go-review.googlesource.com/c/go/+/534097
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>