runtime: fix setting of cpu features for amd64

Because of wrong case of letters, the cpu features flags were not
set properly for amd64.

Fixes #48406.

Change-Id: If19782851670e91fd31d119f4701c47373fa7e71
GitHub-Last-Rev: 91c7321ca49343c86917f071babec8a004ca5c77
GitHub-Pull-Request: golang/go#48403
Reviewed-on: https://go-review.googlesource.com/c/go/+/350151


Trust: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>