The profile.py tries to validate whether the "ip" from the
context register is a kernel ip address or not by comparing
"ip" to the kernel PAGE_OFFSET.

On non-x86_64 architectures, PAGE_OFFSET can be directly used.
On x86_64 architecture, things become more complex.
With 4.17 (latest linus tree), __PAGE_OFFSET_BASE is gone
and replaced with __PAGE_OFFSET_BASE_L5 and __PAGE_OFFSET_BASE_L4
for 5 and 4 level page table respectively.
Running profile.py will have the following error:
    /virtual/main.c:41:18: error: use of undeclared identifier '__PAGE_OFFSET_BASE'
            if (ip > __PAGE_OFFSET_BASE) {
                     ^
    1 error generated.

So, for 4.17, __PAGE_OFFSET_BASE_{L4, L5} should be used.
For 4.16 and some old kernels (e.g., 4.11), which I did not
check earlier one, __PAGE_OFFSET_BASE should be used.
For even older kernels like 4.6, PAGE_OFFSET should be used.
Signed-off-by: Yonghong Song <yhs@fb.com>