runtime: check amd64 microarchitecture level at startup

Make Go runtime throw if it's been compiled to assume instruction
set extensions that aren't available on the CPU.
Updates #48506

Change-Id: Ic4d6696e1cd6b28d389a86fe64e8175ea3ca135a
GitHub-Last-Rev: ba338377f549344b416fbd519576e95ce5d523be
GitHub-Pull-Request: golang/go#48514
Reviewed-on: https://go-review.googlesource.com/c/go/+/351191


Run-TryBot: Martin Möhrmann <martin@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <martin@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Martin Möhrmann <martin@golang.org>