internal/sysinfo: print cpu type from cpuinfo when internal cpu name is empty on...

internal/sysinfo: print cpu type from cpuinfo when internal cpu name is empty on Linux

Supports all linux operating systems. currently tested on x86, mips64le and loong64.

Example output:
$ go test -bench=.*
goos: linux
goarch: loong64
pkg: runtime
cpu: Loongson-3A5000-HV @ 2500.00MHz
BenchmarkSemTable/OneAddrCollision/n=1000         	   19261	     62302 ns/op
    ...

Change-Id: I02db12d70c11327e4625bb6e59f30dfaf37c2db0
Reviewed-on: https://go-review.googlesource.com/c/go/+/508735
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>