For whatever reason,1 the kernel predominantly used on the Pi is compiled from sources that aren't integrated into the mainline Linux kernel, so don't use that.
The Pi fork is available via the Foundation's github repo:
Beware the default branch on the web interface currently seems to be 4.4, but if you look in the "branch" pulldown on the side there are ones for every minor version from 3.8 up to 4.9 (there's 3.2 and 3.6 as well). If you are going to download a zip, you'll have to choose which you want. The one used by Raspbian right now is 4.8.y.
However, if you git it:
git clone https://github.com/raspberrypi/linux
You can then
git branch -a
And checkout/pull whichever one you want.
It's the same source regardless of whether you are building the ARMv6 or ARMv7 version, but you must configure for one or the other. To get the default, I believe for ARMv6
And for v7:
1. The fact that you can download the source and compile it implies it isn't really a licensing issue (although perhaps it is). The fact that Fedora now have a version of their ARMv7 distribution specialized for the Pi but using, AFAICT, the official Linux kernel, and that this kernel still doesn't support things like the SoC's soundsystem, implies some stuff may have been integrated into it from the Raspberry Pi Foundation kernel, and the claim there that getting this "supported in the upstream kernel...is one of the big items on the ToDo list" (who's?) further implies somebody is making an active effort to do that.
I suspect the major reason is a lack of desire, and keeping it separate may be a good thing. But this is all just conjecture and not anything I've investigated. In any case, unlike various other similar ARM boards, The Foundation's kernel source is at least available and kept pretty up-to-date.