Changes include:
  . Add PT_REGS_FP to access base(FP) register in x64
  . Use macros, intead of directly ctx-><reg_name>
    in a few places
  . Let userspace fill in the value of PAGE_SIZE.
    Otherwise, arm64 needs additional headers to
    get this value for kernel.
  . In tools/wakeuptime.py, arm64 and x86_64 have
    the same stack walker mechanism. But they
    have different symbol/macro to represent
    kernel start address.
With these changes, the test py_test_tools_smoke
can pass on arm64.
Signed-off-by: Yonghong Song <yhs@fb.com>