Develop generic implementation of circl/ecc/p384

The packge `circl/ecc/p384` contains optimized code for AMD64 and ARM64, but lacks of a generic implementation using `math/bits` package.

Start by reading the circl/ecc/p384 package and its optimized AMD64 and ARM64 implementations, then compare their operations with the capabilities of Go's math/bits package. The work is done when p384 also has a generic implementation that provides equivalent functionality without architecture-specific code; identify or add relevant tests in the package to verify it.

Written by the indexing model from the issue text.