fix checkptr in (*Mem).Get on darwin on Go 1.22

The unit tests currently fail under race on Go 1.22 on Darwin.

    > go test -race -v -run TestMem

    fatal error: checkptr: converted pointer straddles multiple allocations

    goroutine 35 gp=0xc000104c40 m=0 mp=0x104af4d00 [running]:
    runtime.throw({0x1048d66ee?, 0xc00012c7d8?})
            /opt/homebrew/Cellar/go/1.22.1/libexec/src/runtime/panic.go:1023 +0x40 fp=0xc000067cc0 sp=0xc000067c90 pc=0x1046b8a20
    runtime.checkptrAlignment(0x1048ca639?, 0xa?, 0x104af4d00?)
            /opt/homebrew/Cellar/go/1.22.1/libexec/src/runtime/checkptr.go:26 +0x70 fp=0xc000067ce0 sp=0xc000067cc0 pc=0x1046861e0
    github.com/elastic/gosigar.sysctlbyname({0x1048ca639, 0xa}, {0x10494bca0, 0xc000067e30})
            /Users/ssd/src/gosigar/sigar_common_darwin.go:483 +0xa8 fp=0xc000067d60 sp=0xc000067ce0 pc=0x1048800d8
    github.com/elastic/gosigar.(*Mem).Get(0xc000067e30)
            /Users/ssd/src/gosigar/sigar_common_darwin.go:48 +0x78 fp=0xc000067df0 sp=0xc000067d60 pc=0x10487d2a8
    github.com/elastic/gosigar_test.TestMem(0xc0001349c0)
            /Users/ssd/src/gosigar/sigar_interface_test.go:37 +0x3c fp=0xc000067ed0 sp=0xc000067df0 pc=0x10488242c
    testing.tRunner(0xc0001349c0, 0x104996310)
            /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1689 +0x184 fp=0xc000067fa0 sp=0xc000067ed0 pc=0x10478d1c4
    testing.(*T).Run.gowrap1()
            /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1742 +0x44 fp=0xc000067fd0 sp=0xc000067fa0 pc=0x10478e664
    runtime.goexit({})
            /opt/homebrew/Cellar/go/1.22.1/libexec/src/runtime/asm_arm64.s:1222 +0x4 fp=0xc000067fd0 sp=0xc000067fd0 pc=0x1046f5894
    created by testing.(*T).Run in goroutine 1
            /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1742 +0x5e8

This is because syscall.Sysctl assumes that all return values are C
strings and truncates the final byte if it is NUL.  For Sysctl's that
return integers, attempting to interpret the 7 byte buffer results in
the checkptr violation.

Here, we fix this by using the sys/unix packages SysctlUint64.

sigar_common_darwin.go

@@ -26,6 +26,8 @@ import (
 	"syscall"
 	"time"
 	"unsafe"
+
+	"golang.org/x/sys/unix"
 )
 
 // Get fetches LoadAverage data
@@ -471,21 +473,23 @@ func vmInfo(vmstat *C.vm_statistics_data_t) error {
 
 // generic Sysctl buffer unmarshalling
 func sysctlbyname(name string, data interface{}) (err error) {
-	val, err := syscall.Sysctl(name)
-	if err != nil {
-		return err
-	}
-
-	buf := []byte(val)
-
 	switch v := data.(type) {
 	case *uint64:
-		*v = *(*uint64)(unsafe.Pointer(&buf[0]))
-		return
-	}
+		res, err := unix.SysctlUint64(name)
+		if err != nil {
+			return err
+		}
+		*v = res
+		return nil
+	default:
+		val, err := syscall.Sysctl(name)
+		if err != nil {
+			return err
+		}
 
-	bbuf := bytes.NewBuffer([]byte(val))
-	return binary.Read(bbuf, binary.LittleEndian, data)
+		bbuf := bytes.NewBuffer([]byte(val))
+		return binary.Read(bbuf, binary.LittleEndian, data)
+	}
 }
 
 func taskInfo(pid int, info *C.struct_proc_taskallinfo) error {