crypto/elliptic: store P-256 precomputed basepoint table in source

Store the precomputed P-256 basepoint table in source rather than
computing it at runtime, saving ~88kB from the heap. The flip side
is that this increases binary sizes by ~77kB.

Fixes #44992

Change-Id: Ia5421eae87b41522b0d8cecba051cba1d2ed73db
Reviewed-on: https://go-review.googlesource.com/c/go/+/315189


Run-TryBot: Roland Shoemaker <roland@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>