runtime: add fastrand64

runtime: add fastrand64

Support fastrand64 in the runtime, although fastrand uses wyrand to generate 64-bit random number, it still returns uint32. In some cases, we need to generate a 64-bit random number, the new API would be faster and easier to use, and at least we can use the new function in these places:

src/net/dnsclient.go:randInt()
src/hash/maphash/maphash.go:MakeSeed()
src/runtime/map.go:mapiterinit()

name                 time/op
Fastrand-16          0.09ns ± 5%
Fastrand64-16        0.09ns ± 6%

Change-Id: Ibb97378c7ca59bc7dc15535d4872fa58ea112e6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/400734
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>