Use RdSeed when available, and reduce RdRand load

Summary:
This introduces support for autodetecting and using the RdSeed instruction.

In addition:

• In SeedFast, only 64 bits of entropy are generated through RdRand (256 was relatively slow).
• In SeedStartup, 256 bits of entropy are generated, using RdSeed (preferably) or RdRand (otherwise).

This is a backport of Core PR15250

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4512