cmake: Fix using spirv-tools package on Ubuntu 22.04

When using the `spirv-tools` system package on Ubuntu 20.04 the
`SPIRV-Tools-shared` CMake target, which represents
`/usr/lib/x86_64-linux-gnu/libSPIRV-Tools.so`, is imported and used for
linking and include directories. However, on Ubuntu 22.04 it does not
exist which results in the following error:

```
CMake Error at /builds/SPIRV-LLVM-Translator/CMakeLists.txt:124 (get_target_property):
  get_target_property() called with non-existent target "SPIRV-Tools-shared".
```

Instead, on Ubuntu 22.04 the `SPIRV-Tools-static` CMake target is
imported, which represents `/usr/lib/x86_64-linux-gnu/libSPIRV-Tools.a`,
which suggests the project or package maintainers changed the default
library type being compiled.

This patch updates the `CMakeLists.txt` to check if either of these
targets exists, preferring the shared library, before attempting to get
properties from the target or use it as a link library. If neither
target exists it is treated as a fatal error.

Author: kbenzie

CMakeLists.txt

@@ -120,8 +120,19 @@ if (NOT SPIRV_TOOLS_FOUND)
   find_package(SPIRV-Tools-tools)
   if (SPIRV-Tools_FOUND AND SPIRV-Tools-tools_FOUND)
     set(SPIRV_TOOLS_FOUND TRUE)
-    set(SPIRV_TOOLS_LDFLAGS SPIRV-Tools-shared)
-    get_target_property(SPIRV_TOOLS_INCLUDE_DIRS SPIRV-Tools-shared INTERFACE_INCLUDE_DIRECTORIES)
+    # check for the existance of library targets in the found packages
+    if(TARGET SPIRV-Tools-shared)
+      # use the shared libary target if present
+      set(SPIRV-Tools-library SPIRV-Tools-shared)
+    elseif(TARGET SPIRV-Tools-static)
+      # otherwise fallback to the static library target
+      set(SPIRV-Tools-library SPIRV-Tools-static)
+    else()
+      message(FATAL_ERROR "Found SPIRV-Tools package but neither "
+              "SPIRV-Tools-shared or SPIRV-Tools-static targets exist.")
+    endif()
+    set(SPIRV_TOOLS_LDFLAGS ${SPIRV-Tools-library})
+    get_target_property(SPIRV_TOOLS_INCLUDE_DIRS ${SPIRV-Tools-library} INTERFACE_INCLUDE_DIRECTORIES)
   endif()
 endif()