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.go

@@ -106,6 +106,7 @@ var ARM64 struct {
 	HasSVE2     bool // Scalable Vector Extensions 2
 	HasASIMDFHM bool // Advanced SIMD multiplication FP16 to FP32
 	HasDIT      bool // Data Independent Timing support
+	HasI8MM     bool // Advanced SIMD Int8 matrix multiplication instructions
 	_           CacheLinePad
 }