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_linux_arm64.go

@@ -38,6 +38,7 @@ const (
 	hwcap_DIT      = 1 << 24
 
 	hwcap2_SVE2 = 1 << 1
+	hwcap2_I8MM = 1 << 13
 )
 
 // linuxKernelCanEmulateCPUID reports whether we're running
@@ -112,6 +113,7 @@ func doinit() {
 
 	// HWCAP2 feature bits
 	ARM64.HasSVE2 = isSet(hwCap2, hwcap2_SVE2)
+	ARM64.HasI8MM = isSet(hwCap2, hwcap2_I8MM)
 }
 
 func isSet(hwc uint, value uint) bool {