cpu: add support for detecting cpu features on loong64

Except for lasx, all other features have been implemented
in the Go mainline.

Change-Id: I61a09396ed23d17991b641a1265e952585cb5636
Reviewed-on: https://go-review.googlesource.com/c/sys/+/655355
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>

cpu/cpu.go

@@ -149,6 +149,18 @@ var ARM struct {
 	_           CacheLinePad
 }
 
+// The booleans in Loong64 contain the correspondingly named cpu feature bit.
+// The struct is padded to avoid false sharing.
+var Loong64 struct {
+	_         CacheLinePad
+	HasLSX    bool // support 128-bit vector extension
+	HasLASX   bool // support 256-bit vector extension
+	HasCRC32  bool // support CRC instruction
+	HasLAM_BH bool // support AM{SWAP/ADD}[_DB].{B/H} instruction
+	HasLAMCAS bool // support AMCAS[_DB].{B/H/W/D} instruction
+	_         CacheLinePad
+}
+
 // MIPS64X contains the supported CPU features of the current mips64/mips64le
 // platforms. If the current platform is not mips64/mips64le or the current
 // operating system is not Linux then all feature flags are false.