cpu: add Int8 matrix multiplication instructions CPU feature flag for ARM64

References:
https://github.com/torvalds/linux/blob/5bbd9b249880dba032bffa002dd9cd12cd5af09c/arch/arm64/include/uapi/asm/hwcap.h#L75C9-L75C31

https://developer.arm.com/documentation/ddi0601/2024-03/AArch64-Registers/ID-AA64ISAR1-EL1--AArch64-Instruction-Set-Attribute-Register-1

Change-Id: Ic4e1cf2c23097c7e8695453b6d0b335756d474bc
Reviewed-on: https://go-review.googlesource.com/c/sys/+/595678
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

cpu/cpu_arm64.go

@@ -39,6 +39,7 @@ func initOptions() {
 		{Name: "asimddp", Feature: &ARM64.HasASIMDDP},
 		{Name: "asimdfhm", Feature: &ARM64.HasASIMDFHM},
 		{Name: "dit", Feature: &ARM64.HasDIT},
+		{Name: "i8mm", Feature: &ARM64.HasI8MM},
 	}
 }
 
@@ -146,6 +147,11 @@ func parseARM64SystemRegisters(isar0, isar1, pfr0 uint64) {
 		ARM64.HasLRCPC = true
 	}
 
+	switch extractBits(isar1, 52, 55) {
+	case 1:
+		ARM64.HasI8MM = true
+	}
+
 	// ID_AA64PFR0_EL1
 	switch extractBits(pfr0, 16, 19) {
 	case 0: