cmd/internal/sys: mark windows/arm64 as c-shared-capable

The platform supports c-shared now, so flip this on. I've given this a
small smoke test using [1], and it was able to pass packets and
generally function well. Since [1] uses quite a bit of Go functionality
under the hood, I think it's a decent test that a lot of things that
should be working are working. So this commit enables it.

[1] https://git.zx2c4.com/wireguard-windows/about/embeddable-dll-service/README.md

Updates #46502.

Change-Id: I5c771d033bd20e5ce472c315d7c207bbc1020b4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/326310
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/dist/test.go

@@ -1057,7 +1057,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
  "darwin-amd64", "darwin-arm64",
  "freebsd-amd64",
  "android-arm", "android-arm64", "android-386",
- "windows-amd64", "windows-386":
+ "windows-amd64", "windows-386", "windows-arm64":
  return true
  }
  return false

src/cmd/internal/sys/supported.go

@@ -74,7 +74,7 @@ func BuildModeSupported(compiler, buildmode, goos, goarch string) bool {
  "android/amd64", "android/arm", "android/arm64", "android/386",
  "freebsd/amd64",
  "darwin/amd64", "darwin/arm64",
- "windows/amd64", "windows/386":
+ "windows/amd64", "windows/386", "windows/arm64":
  return true
  }
  return false