Add support for arm64 (aarch64)

Fixes issue #338

src/arch/CMakeLists.txt

-add_library(arch x86_64.cpp)
+if(CMAKE_HOST_SYSTEM_PROCESSOR STREQUAL "aarch64")
+    add_library(arch aarch64.cpp)
+else()
+    add_library(arch x86_64.cpp)
+endif()

src/arch/aarch64.cpp

+#include "arch.h"
+
+#include <algorithm>
+#include <array>
+
+namespace bpftrace {
+namespace arch {
+
+static std::array<std::string, 35> registers = {
+  "r0",
+  "r1",
+  "r2",
+  "r3",
+  "r4",
+  "r5",
+  "r6",
+  "r7",
+  "r8",
+  "r9",
+  "r10",
+  "r11",
+  "r12",
+  "r13",
+  "r14",
+  "r15",
+  "r16",
+  "r17",
+  "r18",
+  "r19",
+  "r20",
+  "r21",
+  "r22",
+  "r23",
+  "r24",
+  "r25",
+  "r26",
+  "r27",
+  "r28",
+  "r29",
+  "r30",
+  "r31",
+  "sp",
+  "pc",
+  "pstate",
+};
+
+static std::array<std::string, 8> arg_registers = {
+  "r0",
+  "r1",
+  "r2",
+  "r3",
+  "r4",
+  "r5",
+  "r6",
+  "r7",
+};
+
+int offset(std::string reg_name)
+{
+  auto it = find(registers.begin(), registers.end(), reg_name);
+  if (it == registers.end())
+    return -1;
+  return distance(registers.begin(), it);
+}
+
+int max_arg()
+{
+  return arg_registers.size() - 1;
+}
+
+int arg_offset(int arg_num)
+{
+  return offset(arg_registers.at(arg_num));
+}
+
+int ret_offset()
+{
+  return offset("r0");
+}
+
+int pc_offset()
+{
+  return offset("pc");
+}
+
+std::string name()
+{
+  return std::string("aarch64");
+}
+
+} // namespace arch
+} // namespace bpftrace