features: add LinuxVersionCode function to obtain LINUX_VERSION_CODE

In order to allow userspace to determine the value used for LINUX_VERSION_CODE
at compile time, expose the constant here.

This is useful when building eBPF objects at runtime.

features/version.go

@@ -0,0 +1,16 @@
+package features
+
+import "github.com/cilium/ebpf/internal"
+
+// LinuxVersionCode returns the version of the currently running kernel
+// as defined in the LINUX_VERSION_CODE compile-time macro.
+//
+// The kernel version and patch level are represented as a single value
+// equal to the KERNEL_VERSION macro from linux/version.h.
+func LinuxVersionCode() (uint32, error) {
+	v, err := internal.KernelVersion()
+	if err != nil {
+		return 0, err
+	}
+	return v.Kernel(), nil
+}