Golang crypto/ecdsa: use variable time ScalarBaseMult/ScalarMult in Verify

We're leaving some perf on the table by doing these operations in constant time. We should use a variable time scalar mult, similar to what we do for crypto/ed25519: https://github.com/golang/go/blob/1a93e4a2cf43b0ded141d33620966bb252cac1bd/src/crypto/internal/fips140/ed25519/ed25519.go#L335

cc @FiloSottile

Comment From: gabyhelp

Related Code Changes

• crypto/elliptic: update P-521 docs to say it's constant-time

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