runtime: restore old mp.fastrand initialization
runtime: restore old mp.fastrand initialization

CL 337350 changed mp.fastrand from a [2]uint32 to a uint64 and changed
the initialization to a single call of int64Hash. However, int64Hash
returns uintptr, so 32-bit systems this always left the most
significant 32 bits of mp.fastrand initialized to 0. The new code also
did not protect against initializing mp.fastrand to 0, which on a
system that does not implement math.Mul64 (most 32-bit systems) would
lead fastrand to always return 0.

This CL restores the mp.fastrand initialization to what it was before
CL 337350, adjusted for the change from [2]uint32 to uint64.

Change-Id: I663b415d9424d967e8e665ce2d017604dcd5b204
Reviewed-on: https://go-review.googlesource.com/c/go/+/383916
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go