[libclc] Give a helpful error when an unknown target is requested (#111528)

I just tried using LLVM backend names here e.g. NVPTX but libclc want's
targets more like triples. This change adds a mesasge to tell you that.

Before you got:
```
 libclc target 'AMDGCN' is enabled
 CMake Error at CMakeLists.txt:253 (list):
   list index: 1 out of range (-1, 0)

 CMake Error at CMakeLists.txt:254 (list):
   list index: 2 out of range (-1, 0)

 Configuring incomplete, errors occurred!
```
Now you get:

```
 CMake Error at CMakeLists.txt:145 (message):
   Unknown target in LIBCLC_TARGETS_TO_BUILD: "AMDGCN"

   Valid targets are:
   amdgcn--;amdgcn--amdhsa;clspv--;clspv64--;r600--;nvptx--;nvptx64--;nvptx--nvidiacl;nvptx64--nvidiacl;amdgcn-mesa-mesa3d
```
Some of the targets are dynamic based on what is installed, so spirv
isn't here for me because I don't have llvm-spirv installed yet.

So this is not perfect but it's an improvement on the current behaviour.