[libclc] Move sign to the CLC builtins library

[frasercrmck]

This commit moves the sign builtin's implementation to the CLC library.
It simultaneously optimizes it (for vector types) by removing
control-flow from the implementation.

The __CLC_INTERNAL preprocessor definition has been repurposed (without
the leading underscores) to be passed when building the internal CLC
library. It was only used in one other place to guard an extra maths
preprocessor definition, which we can do unconditionally.

[frasercrmck]

CC @tstellar @fooishbar who might be able to account for the Mesa/SPIR-V target?

[tstellar]

So is the sign function currently implemented in the OpenCL headers?

[frasercrmck]

So is the sign function currently implemented in the OpenCL headers?

The sign function for SPIR-V/Mesa is currently implemented by libclc here. With this change, sign now calls __clc_sign which has the same implementation. We just need to do a bit of extra inlining of CLC functions into OpenCL functions to maintain more or less the same SPIR-V.

[fooishbar]

cc @karolherbst @airlied who are better contacts for CL in Mesa these days

[frasercrmck]

@arsenm I've just updated this PR. It still moves the sign implementation to the CLC library but now also optimizes it for vector types. There's no control flow nor scalarizing for vectors. I trust this will be a good enough default for the scalar form (on GPUs) though I suppose CPUs may prefer the old control flow form. I doubt any would prefer the old vector form, though.

alive2 seems to be happy with the transformation (for half2): https://alive2.llvm.org/ce/z/RZ8Dc4

Let me know what you think.

[frasercrmck]

I'll look into undoing the CMake changes and building the dependent CLC functions for SPIR-V targets later on.

[frasercrmck]

ping, thanks