runtime: accept restartable sequence pcdata values in isAsyncSafePoint

If the pcdata value indicates a restartable sequence, it is okay
to asynchronously preempt (and resume at the restart PC). Accept
it in isAsyncSafePoint.

Fixes #47530.

Change-Id: I419225717c8eee5812f3235338262da5895aad0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/340011


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Austin Clements <austin@google.com>