m1cpu: handle kIOMainPortDefault for macOS < 12 (#6)

`kIOMainPortDefault` was renamed from `kIOMasterPortDefault` in macOS
Monterey. This patch makes sure m1cpu can be built on Big Sur, too.

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>

cpu.go

@@ -3,10 +3,15 @@
 package m1cpu
 
 // #cgo LDFLAGS: -framework CoreFoundation -framework IOKit
+// #include <AvailabilityMacros.h>
 // #include <CoreFoundation/CoreFoundation.h>
 // #include <IOKit/IOKitLib.h>
 // #include <sys/sysctl.h>
 //
+// #ifndef MAC_OS_VERSION_12_0
+// #define kIOMainPortDefault kIOMasterPortDefault
+// #endif
+//
 // #define HzToGHz(hz) ((hz) / 1000000000.0)
 //
 // UInt64 global_pCoreHz;