crypto/internal/fips/ecdsa: add CAST, PCT, DRBG, and FIPS 186-5 references

crypto/internal/fips/ecdsa: add CAST, PCT, DRBG, and FIPS 186-5 references

The previous CL focused on moving the implementation as-is, while this
makes it FIPS-compliant.

For #69536

Change-Id: I75fa56c7e13ba20246bacf9fda4599c9f25a1c63
Reviewed-on: https://go-review.googlesource.com/c/go/+/628678
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>