R600/SI: Add intrinsics for various math instructions.

These will be used for custom lowering and for library
implementations of various math functions, so it's useful
to expose these as builtins.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211247 91177308-0d34-0410-b5e6-96231b3b80d8