message: Fix TestRegisterVariables for clang/arm
  Clang rejects __attribute__((regparm)) when targetting arm. The default
  calling convention passes arguments in registers anyway, so we can just
  remove them in this case.
diff: #include <stdio.h>
  +#if defined(__arm__) || defined(__aarch64__)
  +// Clang does not accept regparm attribute on these platforms.
  +// Fortunately, the default calling convention passes arguments in registers
  +// anyway.
  +#define REGPARM(N)
  +#else
  +#define REGPARM(N) __attribute__((regparm(N)))
  +#endif
  +
  struct bar {
  int m1;
  int m2;
  };
  -void f1(int a, struct bar *b) __attribute__((noinline)) __attribute__((regparm(2)));
  +void f1(int a, struct bar *b) __attribute__((noinline)) REGPARM(2);
  void f1(int a, struct bar *b)
  {
  b->m2 = b->m1 + a; // set breakpoint here
  }
  -void f2(struct bar *b) __attribute__((noinline)) __attribute__((regparm(1)));
  +void f2(struct bar *b) __attribute__((noinline)) REGPARM(1);
  void f2(struct bar *b)
  {
  int c = b->m2;
  