crypto/elliptic: use a const string for precomputed P256 table
Const strings can be marked readonly. This is particularly
important for this relatively large table (88kb).
This is a follow-up to CL 315189.
The generation script is a bit awkward. It needs access to crypto/elliptic
internals, but also needs to be package main. Work around this by
exporting those internals with the "tablegen" build tag.
This requires changing the function signature at the Go-asm bridge.
As long as we're here, shrink the point argument type as well;
the net result is three fewer words of params.