internal/cpu, runtime: make linux/loong64 HWCAP data available

internal/cpu, runtime: make linux/loong64 HWCAP data available

This can be used to toggle runtime usages of ISA extensions as such
usages appear.

Only the CRC32 bit is exposed for now, as the others are not going to be
utilized in the standard library for a while.

Change-Id: I774032ca84dc8bcf1c9f17558917315af07c7314
Reviewed-on: https://go-review.googlesource.com/c/go/+/482416
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: xiaodong liu <teaofmoli@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>