[libc++] Replace modulus operations in std::seed_seq::generate with conditional checks.

Abseil benchmarks suggest that the conditional checks result in faster code (4-5x)
as they are compiled into conditional move instructions (cmov on x86).

Reviewed By: #libc, philnik, Mordante

Spies: pengfei, Mordante, philnik, libcxx-commits

Differential Revision: https://reviews.llvm.org/D125329