Tweak documentation.

1. Having "get started", "get involved", and "hacking" makes it hard to find
   how to send patches, so add a link from "get involved" to "hacking".
2. Remove an almost 5 year old note on the test running meachanism changing
   soon.
3. Let "hacking" link to the LLVM developer policy.
R600: Mostly remove remaining AMDIL intrinsics.

Delete all unused ones, and add new AMDGPU named intrinsics for
the ones that are. Handle the old AMDIL names for comptability (although
remove their GCCBuiltin names) and add tests since there weren't any
for these before.