internal/asan: match runtime.asan{read,write} len parameter type

internal/asan: match runtime.asan{read,write} len parameter type

The len parameter runtime.asan{read,write} is of type uintptr. Match its
type in Read and Write.

For #64611

Change-Id: I0be278c38a357e600521ced87c0e23038a11e8a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/572755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>