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

[David Spickett 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
 targets is welcome.
 
-Compiling and installing with Make
-----------------------------------
+Compiling and installing
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DavidSpickett wrote:

I didn't know there was such a variable so, yes I'll mention that.

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