Fix Jacobi benchmarks and other benchmark improvements

Summary:

• Rename bench_internal variables

The _x and _y suffices are confusing; they don't actually correspond
to X and Y coordinates. Instead replace them with arrays.

• Randomize the Z coordinates in bench_internal

Also increase the number of fe inputs.

• Make jacobi benchmarks vary inputs

Also make the num_jacobi benchmark use the scalar order as modulus,
instead of a random number.

• Add benchmark for secp256k1_ge_set_gej_var

This is a backport of secp256k1 PR797

Test Plan:

ninja bench-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7612