runtime: check amd64 microarchitecture level at startup
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 <54669547a225ff20cba8b75a4adca540eef25858@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Martin Möhrmann <54669547a225ff20cba8b75a4adca540eef25858@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Trust: Martin Möhrmann <54669547a225ff20cba8b75a4adca540eef25858@golang.org>

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