[dev.boringcrypto] crypto/tls: permit P-521 in FIPS mode

[dev.boringcrypto] crypto/tls: permit P-521 in FIPS mode

While BoringCrypto has a certification for P-521, the go code disallows
certificates with it. This change permits those certificates to be used.

Change-Id: I451c91a845f22ff0e4c3e922eaf8bf82466e80ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/343880
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>