crypto/tls: allow P-521 in FIPS 140-3 mode and Go+BoringCrypto

crypto/tls: allow P-521 in FIPS 140-3 mode and Go+BoringCrypto

Partially reverts CL 587296, restoring the Go+BoringCrypto 1.23 behavior
in terms of supported curves.

Updates #71757

Change-Id: I6a6a465651a8407056fd0fae091d10a945b37997
Reviewed-on: https://go-review.googlesource.com/c/go/+/657095
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>