crypto/rand: Legacy RtlGenRandom use on Windows [1.20 backport]

[gopherbot]

@rolandshoemaker requested issue #53192 to be considered for backport to the next 1.20 minor release.

@gopherbot please open backport issues, this reduces the impact of a security issue.

[gopherbot]

Change https://go.dev/cl/545356 mentions this issue: [release-branch.go1.20] crypto/rand,runtime: switch RtlGenRandom for ProcessPrng

[gopherbot]

Closed by merging 1bd7657 to release-branch.go1.20.

[dmitshur]

This cherry-pick doesn't apply to Go 1.20 because it supports older Windows versions where ProcessPrng isn't available. The change was rolled back in CL 545995. Re-closing as "not planned".