[libclc] [libclc] Update build instructions in readme (PR #111369)

[Fraser Cormack via cfe-commits]

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@@ -23,29 +23,41 @@ generic implementations of most library requirements, allowing the target
 to override the generic implementation at the granularity of individual
 functions.
 
-libclc currently only supports the PTX target, but support for more
+libclc currently supports NVPTX, AMDGPU and SPIRV targets, but support for more
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frasercrmck wrote:

This is definitely better, but I'm not sure how we'd classify the `clspv` and `clspv64` targets as they don't conveniently fit into the "LLVM target" model. Same goes with SPIR-V, actually, as I pointed out further down. Saying "We support the SPIRV targets" might conflate the two notions of LLVM target and libclc target.

https://github.com/llvm/llvm-project/pull/111369