[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
 targets is welcome.
 
-Compiling and installing with Make
-----------------------------------
+Compiling and installing
----------------
frasercrmck wrote:

Do you think we need to mention the use of `LIBCLC_TARGETS_TO_BUILD` to control which targets are built? The default is "all".

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