crypto/internal/fips140/nistec/fiat: optimize P256mul on riscv64

[newborn22]

This PR optimizes the core p256Mul operation for P256 elliptic curve cryptography on the RISCV64 architecture.

Key Optimization:

1. Leverages the structural characteristics of the P256 modulus (p = 2²⁵⁶ - 2²²⁴ + 2¹⁹⁶ + 2⁹⁶ - 1). Converts the multiplication operation acc0 * p into shift operations, replacing 64-bit multiplications in the generic Go implementation and avoiding the overhead of big-integer multiplication.

2. Direct Register Operations: Uses registers for data transfer throughout the implementation, eliminating the extra overhead of stack-based data transfer in the Go-compiled version.

Unit Tests: TestP256Mul was implemented to verify the correctness of assembly implementation.

Benchmark: in src/crypto/internal/fips140/nistec/fiat/benchmark_test.go

Peformance:
goos: linux
goarch: riscv64
pkg: crypto/internal/fips140/nistec
cpu: SG2044
│ p256mul_base.riscv64.stats │ p256mul_opt.riscv64.stats │
│ sec/op │ sec/op vs base │
ScalarMult/P256-30 564.7µ ± 5% 491.8µ ± 6% -12.91% (p=0.002 n=6)

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Filippo Valsorda:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Weihong Qiu:

Patch Set 1:

(1 comment)

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