runtime, syscall: use pointer types on wasmimport functions

runtime, syscall: use pointer types on wasmimport functions

Now that we support pointer types on wasmimport functions, use
them, instead of unsafe.Pointer. This removes unsafe conversions.
There is still one unsafe.Pointer argument left. It is actually a
*Stat_t, which is an exported type with an int field, which is not
allowed as a wasmimport field type. We probably cannot change it
at this point.

Updates #66984.

Change-Id: I445c70b356c3877a5604bee67d19d99a538c682e
Reviewed-on: https://go-review.googlesource.com/c/go/+/627059
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>