[release-branch.go1.20] crypto/rand,runtime: revert "switch RtlGenRandom for ProcessPrng"

[release-branch.go1.20] crypto/rand,runtime: revert "switch RtlGenRandom for ProcessPrng"

This reverts CL 545356.

Reason for revert: 1.20 still supports Windows versions before
ProcessPrng was introduced.

Change-Id: I224b8c4e7d0ca9ad5e733819b24dd92d14e61ab8
Reviewed-on: https://go-review.googlesource.com/c/go/+/545995
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>