Test RDRAND on x86 / x86_64 targets

[josephlr]

Depends on #471

Now when running cargo test we see the additional rdrand tests:

running 11 tests
test error::tests::test_size ... ok
test rdrand::tests::fill_large ... ok
test rdrand::tests::fill_zero ... ok
test tests::fill_large ... ok
test tests::fill_zero ... ok
test rdrand::tests::multithreading ... ok
test tests::multithreading ... ok
test rdrand::tests::fill_small ... ok
test tests::fill_small ... ok
test tests::fill_huge ... ok
test rdrand::tests::fill_huge ... ok

I think the main question here is if we should unconditionally run these tests like we did before, or if we should only run them if the "rdrand" feature is set.

[newpavlov]

As I wrote here, this probably can be done a bit better.

[josephlr]

Closing to keep conversation in #471