RFR: 8186780: clang-4.0 fastdebug assertion failure in os_linux_x86:os::verify_stack_alignment()

[Martin Buchholz]

There sure seems to be a lot of confusion about how many stack frames we're
getting at the hot end of the stack, e.g.

  register intptr_t **fp __asm__ (SPELL_REG_FP);

  // fp is for this frame (_get_previous_fp). We want the fp for the
  // caller of os::current_frame*(), so go up two frames. However, for
  // optimized builds, _get_previous_fp() will be inlined, so only go
  // up 1 frame in that case.
  #ifdef _NMT_NOINLINE_
    return **(intptr_t***)fp;
  #else
    return *fp;
  #endif